B-negative was a Human blood type.

This blood type was one of the two (the other being AB-positive) remaining blood types Loomis was required to get for Damron, in order for the Xindi-Reptilian to synthesize a virus for their bio-weapon, which required samples of all eight types of Human blood to be properly calibrated. Once Loomis provided Damron with a patient having this blood type, he would have been paid double the bounty they originally offered him.

Captain Archer told Loomis and T'Pol that his blood type was B-negative, and hoped to use this fact to his advantage in stopping the Xindi. (ENT: "Carpenter Street")

Julian Bashir also had type B-negative blood. He shared that information in 2373, following a blood screening to prove he was who he said he was. (DS9: "In Purgatory's Shadow")

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