Have you tried adding a rock for the blenny to perch on???? In my experience, having a rock to perch on provides a sense of security for the blenny.
Also tying a piece of seaweed onto that same rock or pvc fitting could help. Again, in my experience blennies love mowing down algae. Giving the fish a somewhat familiar setting can promote better health. Thats my $0.02.
Im going to assume your parameters are in good shape by the looks of the other fish & some good algae growth on the fittings. Hope it helps. Good luck