For one, the binding is different. The limitations of a blood pact are unknown, and it is probably breakable, unlike the Unbreakable Vow (duh)

^This. Probably. IDK.

I think the only difference is that in the blood pact, the blood of both the parties are intermixed when doing the pact and the consequences of blood pact are unclear, whereas in the Unbreakable vow, you die when you break it

^This probably.