No, it's never been an expectation, just that recently I'd heard (and seen for the higher level interviews) that everyone had gone to black for a noticeable number of years now such that I wondered if you would stand out too much if you didn't do it now. But it sounds like that might be fading away, woo!
The people I know who conduct interviews say they largely don't care what you wear as long as it doesn't stick out unfavorably somehow. Skirt or pants, flats or heels, exact suit cut or material are all things they purportedly don't even notice. One person I know who's been part of admissions for the big medical school here did say re: pants or skirts that the most important thing is to wear whatever is actually flattering on you and will be comfortable. If you have a wide stride and a skirt is going to trip you up, don't wear one. If you're like me and most slacks make you look hobbit-like, maybe don't do that. Same with the shoes, because they make you walk all over and people definitely notice when a candidate isn't comfortable in her shoes and has a rough time. This isn't the occasion to be breaking new ground with your wardrobe.
With the resident and fellow candidates I've seen a pretty even pants/skirt split and most wear very low heeled but not completely flat shoes. A few in each group will have high heels and a few will have flat-flats. The women in skirts do not wear pantyhose as far as I've ever seen but I haven't been close enough to tell on all of them. Not all of them wear collared dress shirts either, I've seen quite a few other blouses (albeit solid colors always) and I think I'll try it for myself since I am not a shape that looks good in a regular dress shirt.