i got this in my tankDry rock or live rock?
Fishless cycle with ammonia, or did you use a food source (table shrimp, fish food)? Or did you cycle with a fish?
Is the Blenny the only fish ever to go into the tank? (I know it is the only one now, but did you have any die on you previously?)
Assuming multiple test kits have shown that sky high Nitrate, besides using some heavy water changes to knock the level down you need to figure out what the root cause was for this. That will in part determine what your courses of action to take after diluting the current level. Get yourself a large clean garbage can (45 gallons or greater) so that you can make some sizable batches of new seawater.