Although Kosher Sushi is already extremely popular in Orthodox circles, I have not yet seen a definitive answer as to what is the proper blessing to be made? Just for conciseness, Sushi is steamed rice rolled with raw/cooked fish/vegetables/seaweed.

One makes "Borei Minei Mezonot" on a dish that is primarily rice. After eating, the bracha is "Borei Nefashot" (Shulchan Aruch 208 7).