As much as I love Buffy and as much as I respect Joss, I would have to say that he missed the target completely here. He may know Buffy, but I know martial arts, and Natasha is extensively trained in just about every form there is, including the Russian martial art of Combat Sambo (don't laugh, that IS its name, and from the demonstrations that I have seen, devastating is too small a word to describe it) That training would certainly include numerous techniques designed to deal with much stronger, and even faster, opponents. Buffy is an excellent street fighter, but we saw in her test against the Watchers' fighter, she has no real knowledge of any formal art. And don't think that, just because she was able to beat the Watcher that she could handle a properly trained martial artist of Natasha's caliber.
Buffy has a fatal flaw that the Black Widow would recognize immediately and exploit to deadly advantage: over confidence. We saw it in Season 2 in "School Hard," when she underestimated Spike with almost fatal results, we saw it again the first time that she fought Glory, and we saw it twice more in Season 7 with both the Ubervamp and Caleb. We even saw it in Season 4, when a perfectly normal vampire pulled an elementary Hapkido move and staked her with her own stake. In each case, she could have died for her mistake. Sure, she managed to learn from those mistakes each time, come back, and win in Round two or three, but, as soon as she DID win, the old "I'm the baddest pit bull in the kennel" attitude came right back. She has never learned that her over confidence is her greatest weakness.
Natasha is not the kind of fighter who would overlook something so obvious, and she is not the kind of fighter who wouldn't immediately go for the kill the first time the opportunity presented itself. And, remember, when Buffy fights Natasha, she won't have Joyce with a fire axe or a commando boyfriend to save her hash for her. If Buffy tangled with Natasha, there would be no fresh, warm cookies for Angel, Spike, or anybody else. There would just be Buffy, wondering where that freight train came from.
Get out the butter and jelly, folks, cause we've got Slayer toast on the menu today.
Sorry, Joss.