how long have you had your blenny? may be just not used to your tank yet. i didn't have that problem with mine, but we all know that sometimes fish just decide not to eat. i've never seen mine eat any prepared food though. if i didn't have algae in my tank, i think he'd starve. but, he is always fat as can be. sadly he recently stopped eating hair algae and now seems to eat exclusively diatom algae, which is too bad since I have a problem with hair algae, not diatom.
as to the garlic, do as rye suggested, chop up the garlic really well. I'd then put the garlic in a little water and let it soak for a few days (less time would probably be fine), then strain out the garlic pieces. now you have a realitively chunk-free liquid that will be very potent with garlic. use that to soak your food in.