A Blisk Mutant was result of a Blisk spore that infected a host. It was a heavily-armored organic life-form (which was usually a mutated human).
The first Blisk Mutant appeared in the Tunnel network in Albion. The mutant was Ivan Oranchov who shot a container of Blisk spores, which then mutated him into the first Blisk Mutant.
In order to take down a Blisk Mutant, use the Anal probe to disintegrate the Blisk Possession, then finish the damaged Possession down with the Disintegrator Ray or the Zap-O-Matic.