What kind of radiation would an anti-matter blast produce?

You don't have an anti-matter blast, you'd combine matter and anti-matter to have a blast. If the device were 100% efficient, every particle meeting its anti-particle, you'd get high energy photons only, like gamma and X rays.

The problem with anti-matter (apart from the humungous amount of energy to produce it) would be storage. We could not let it touch the walls of the container.

I would say anti-matter radiation, becuase it is anti-matter, or mabye even dark matter. Hmm, something for me to ponder.