The dictionaries say in for being inside or close to the edge of something. And on being on top of something even if that something is inside something. So this may explain why both in and on are acceptable for a blood test, that deals with blood cells within a vein or inside a Petri dish and cells with plasma with abnormalities both within the fluid and on the cells. Possibly the report may have started of as saying in the blood tests and when eventually reaching the patient it may have changed to on the blood tests. Especially if passed from laboratory to patient by mouth to transcribed by typists more than once.

I think both prepositions are ok, but I don’t love the construction to be honest. I think I’d probably just say in your case ‘…. abnormal blood tests or ultrasounds, but a few of them don’t.’ Also you can reverse the order: ‘blood test or ultrasound with/having an abnormal finding’

For my part, I think "on a blood test" sounds best. However, I could be persuaded either way. I think perhaps both are acceptable.