I'm afraid I'm going to need a photo. Also, I can't think of any smasher that large from Central America (except Hemisquilla which is not black). Given the size, my first guess would be a Pseudosquilla ciliata from the Atlantic, but that is a spearer. The stomatopods from Nicaragua are not well known, but they should be pretty similar to what occurs in Costa Rican and Mexican collections.
Did this animal come from a collector? You have me really curious.
Roy