What kind of blenny are you considering? Most blennies I've seen either pick food from the water column or off rockwork, as opposed to digging in the sand. (The engineer blenny digs, but I'm not sure it's eating anythign when it does. I think they're just digging holes to live in.)
If you ahve cruched coral and are looking for something to stir it and eat stuff out of it, you might do better with conchs or nassarius snails. I think crushed coral is too tough on the sand sifting gobies' mouths.
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