It might mean "blood certainly", e.g.
1. Blood sure is red = blood certainly is red.
Or a coinage that means "as sure as blood":
2. The pact between us is blood-sure = the pact between us is sealed in blood.
Or a typo for "bloody sure" ("bloody" is a mild and slightly old-fashioned British intensifier):
3. I'm bloody sure that didn't happen = I am very sure that didn't happen.
Do you have more context?