I've read that in the wild they are cleaners, but seldom exhibit the behavior in captivity.
I have one in a 37 gallon with a Baggai cardinal, royal gramma, and a cinammon clown an anemone, clam, and a few corals. I had a sixline wrasse with him also, but he met his demise at the intake of my protein skimmer pump. He has squared off agains the clown if one got in the other's way, but I haven't seen any actual contact: just threatening posture. He did literally steal a piece of flake food from the tentacles of the long tentacle anemone today.
Others on this site will tell you horror stories of them eating corals and sometimes attacking fish, but I have not seen that with mine. I do advise that you keep and eye on them.
I like mine, but in some respects I wish I hadn't gotten him. He hides most of the time (except for night time and feeding), and I am afraid to add any other shrimp in with him for fear he'll make them into a very expensive meal.
I your case, it sound like the blenny wouldn't get out of the way, so the shrimp took care of business. Maybe the blenny didn't observe the shrimp as any kind of threat. Hopefully he'll learn.
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