The felblood elves are a group of Kael'thas Sunstrider's most loyal followers, who were rewarded by Kil'jaeden with the gift of being allowed to gorge themselves on demonic blood for days for remaining in Outland, even as many of his blood elves retreated through the Dark Portal.
In addition to bearing runic tattoos, they have developed various demonic features, such as unnatural skin and hair colors, horns, and what appear to be small, black-feathered wings — as well as cracks and openings in their skin, which reveal green fel energy glowing from within. Their cuspid teeth are prominent and drape idly out of their mouths during speech, giving them a vampire-like appearance. Unlike the Wretched, who lose most of their hair, felblood elves seem to bypass this aspect of corruption.
Fel elf is the model name used in the game's files. It is unclear if it's an actual race name or just a description. If it is the race name, that would make the felblood more of a faction term or a nickname. Felblood has also been used to describe other demonic creatures as well, as well as the blood of terrorfiends. Like stated above, the blood elves are siphoning energy from Suspended Terrorfiends, so the fact that a terrorfiend's blood is called Felblood, and these elves are also called Felblood, makes sense.