I finished my blackwork sampler from the class at the Renaissance Faire.

I have worked needlepoint and cross-stitch before, so the fact that you work blackwork motifs in segments was not entirely unfamiliar to me. You want to do this because if you work each motif, say the little "wishbones" in my sampler, on its own, the thread will lie in different directions. (For knitters, this is the same principle behind not only the obvious K2tog vs. SSK, but also in the more subtle variations on how to work the SSK, the loops lying differently depending on the way that it is worked.) So with blackwork, basically, you work all of the little dashes along one edge of your pattern, turn the work around, do the next line of dashes, perhaps the angled ones this time, turn the work, do the next segment, and so on. Medallion motifs and so on are on the same principle, although with more pieces of the pattern worked before you go on to the next step.
The class was pretty good, especially because I was the only one signed up that day and thus got the instructor's undivided attention. For your thousand pence (because, marry, it's the Ren Faire), you get a square of 11-count Aida cloth to work practice stitches, needle and thread, the sampler backing in 18-count Aida (a bookmark kit by Charles Craft from Jo-Ann's), and a hand-out, plus of course the instructor for an hour to get you started.
There are actually two different kinds of blackwork: one is the counted kind, worked in short, straight segments which result in a geometric angularity to the design, and free-stitched black work, with a more curvilinear look, and which is really simply embroidery in a single color, usually black.
The first picture is my sampler as it was when I came home:

There are two separate strands of thread because the instructor had me stop working the second angled leg, and start adding in the last straight leg, so that I could see how the pattern would look.
I worked the second border and then picked a center pattern that would fit between the two.
I had a very difficult time figuring out how to read the chart. It has numbers all over it, obviously some sort of sequence, but with numbers missing. I had had a little introduction to these charts from the instructor at the Renaissance Faire, but in my case a little education was probably worse than none at all. Although there is plenty of information on how to do blackwork on the internet, these numbered charts don't seem to be standard, and there is apparently nothing on how to read them. After no little time of trying to understand what went where and how, and not getting it at all, I gave up in frustration and went off to do something else. Some time later, I thought of my old "Golden Hands" set, and looked up blackwork. There was a single two-page spread, which was actually not much help as the project was "easy to work from the photograph" and had no chart at all, but the diagrams somehow sort of began to make sense, and so I picked up the numbered chart again.
Apparently -- and someone please correct me if I'm wrong! -- you simply assume some of the numbers, that some are, as it were, "innies" and some "outies" and you only get the "outies" numbers. Believe it or not, this way of looking at it actually helped, and while I may not have been working it "correctly", I got into the swing of it fairly quickly after that. It certainly looks like the illustration! so I can't be too far off.

Here I have worked the first half of the center motif -- again, you work part of it to the end, then turn around and work your way back, sometimes three or four times. The "wishbone" border took five passes, although (amusingly) the more complicated lozenge pattern only two, but with back-stitches. Here I accidentally left off the extra lines on the first star and didn't notice until two or three along, so I left it in as a "design element", planning to make the last one to match.

But then when I got to the end of the first pass, I found that I had badly estimated the spacing, and it was unbalanced enough to disappoint me greatly, so I picked it all out and did it again, also fixing the first star. "If it's worth doing", and all. I got this far the second time when I saw the mistake which positively leaps out at me now. First person to spot it gets a prize -- oh, I know, two virtual weeks at the Royal School of Needlework!
So I finished the thing, and carefully snipped the black thread in the middle of the mistake and picked out four or five stitches on either side of it, and reworked them with new thread. You'd never know.

Here is the back, in all its sorry glory. The border at the bottom is the one I worked at the Faire -- you can see that for one thing, once I got home I didn't remember in what order I'd done the parts of the "wishbones", and for another, it was much better the second time regardless, and I got into a good rhythm. (The wobbly lines in the top border are the woven-in ends, and at the right is when I careened off the reverse for a moment until I got my balance again.) I found that it was a lot like knitting lace, that I was kind of humming to myself "over two, under three -- over two, under three -- over two, under three" on down the row. The lozenge motif started off very awkwardly and the back was all over the place, but it actually didn't take me very long to see how it was fitting together -- I'm still not at all sure I understand the numbered chart, but after a while I didn't really need it -- all that knitted colorwork stood me in good stead, I guess. The back of the blackwork here is the second try, of course, so it looks much tidier.
The instructor recommended Donna Kooler's Encyclopedia of Needlework for an introduction to blackwork as well as dozens of other different kinds of embroidery, and Mary Gostelow's Blackwork and Rosemary Drysdale's The Art of Blackwork Embroidery for more in-depth stitches and patterns. Online, the Blackwork Embroidery Archives has many lovely patterns for free, but little how-to information for the utter novice.
And because I can't resist, here are some contemporary examples of blackwork:

Jane Seymour, by Holbein, ca.1537. Rather restrained blackwork at the cuff edges of her smock. I wonder if the little black dashes around the neck are some kind of blackwork edging?

Jane Seymour again, by the workshop of Holbein (apparently a copy of the first portrait but with a different smock and undersleeves). Even more restrained blackwork than the other, but look at that pleating on the undersleeves!

Bess of Hardwick by a follower of Hans Eworth, ca.1550s, and no, I don't know why it says "Maria Regina" as this certainly isn't the pinch-faced Mary. Red blackwork, and lots of it -- even on the ruffles!

The only commoner here, Mrs. Pemberton, whoever she is, by Holbein, ca.1535-40. A pretty blackwork collar and cuffs on her smock. Someone has worked out a chart for this smock and made a lovely copy.

Mary Tudor, by someone called "Master John". The blackwork is not especially clear in this image, but it is apparently the freehand kind, not counted like all of the above examples.