We seem to have officially decided that the culprit for the blood was the multivitamin. It turns out that the insurance won't cover vitamins, but the new one is less expensive than the one we were buying before, so it's not that big a deal. Having absolved solid food of any responsibility for blood, we were cleared to start introducing new solids again.
Today we tried avocado. We're working alphabetically through the fruits (started with apple; banana will be next; apparently skipping apricot). First I let Blaise sort of suck on a chunk of avocado while I held it, but she kept grimacing. I thought she might not like that approach to eating, so I mashed some up and gave it to her on a spoon. She took two tiny bites, grimacing at both, and then gagged and vomited. Not just spit up, actual vomit, which she almost never does. Fearing that I had somehow induced an instant oral aversion, I cleaned her up and then tentatively tried some rice cereal. She scarfed that down as usual. I guess she just doesn't like avocado. We'll try again with it in a little while, but maybe it's time for banana.