血案 (xue’an) is an informal term for a murder or massacre (the more formal, common term is 谋杀, mousha). It literally means “blood case.”
Why it is blocked: Obviously, it is a reference to violence, though at first glance, it doesn’t seem any more bloody than mousha, which is unblocked. However, xue’an seems to be a phrase often used to describe the 1989 Tiananmen crackdown. 屠杀 (tusha) is another word for massacre that is blocked for similar reasons.
Note: As 血案 isn’t actually an entry in Wikipedia, I only stumbled upon it after noticing that 一个馒头引发的血案, a parody video of Chen Kaige’s flop The Promise, was blocked.
