Rainbow Six Vegas Black Edition DLC really did appear on Xbox Live. It was brief, but it was there. But now, as you have already found if you've attempted a download over the past day or so, it is most certainly not there. The good Major offers little help on his blog, saying only: "This has been pulled. I'll let you know when it makes a return." Wow, cryptic.
There are, of course, plenty of delightful theories as to why the pack pulled a Judy on Family Matters. Some think that it got on to Marketplace too early, some think a critical bug was discovered. We like to think that Ubisoft was so racked with guilt over charging $10 for five maps (when that amount previously got you five maps and two new modes) that they pulled it. We admit that's not really likely. We'll be sure to update this post when it triumphantly returns.