Thanks to all for the kind words..
I realized today just how much of the time I look at my tank I spent waiting for our blenny to come out and entertain us.
I wish I knew what killed him - he looked healthy, no aparent diseases or swim bladder problems. He was choking on his food for quite a while, and it seemed to be getting worse, so possibly a throat cancer or inflamation?
He quit moving his eyes around towards the end, and one of them looked empty.. I don't know how to explain it. I don't think he was blind because he didn't seem to have trouble finding his way around. I can't help but wonder if it was the flatworms... please do yourself a favor and treat these asap when you see them.. flatworm exit would have been fatal at the populations I had, so I was stuck buying a nudibranch, which I didn't want to do until we move to the new house.
The water seemed OK, though we don't test for phosphates- just ammonia, nitrates, trites, and ph. The other fish seem ok... The coral had started drooping a month or so ago and so we resumed religious water changes.
I hope there was nothing else I could have done to save our little baby!