i actually found this really interesting and had to think for a minute. as the daughter of nurses and a natural science geek i came up with the answer.
theoretically speaking, yes she could. there are small traces of iron that exists in blood. that is why blood has a metallic smell to it. though its possible , that the amount of metal in a person is not enough to sense/detect it. if thats the case, then no Suyin (nor Lin or any metal bender) could bend it.
i’m gonna just assume this is the first time she had ever encountered liquid metal and whats more the metal was not in plain sight - she had to take a deep breath and focus more than she would normally. after a brief pause though she was able to feel the metal and bend it out. (her mercury bending was actually really cool! her movements brought to mind water bending, she had to adapt her posture to be able to bend it, which shows she is very flexible and a very capable bender, her dancing clearly helps her with this.)
So, she would have to focus very, very hard to be able to find metal in blood. it would come down to whether or not the minimum amount of metal is present for a master metal bender to be able to bend/even detect its presence.