The following is written from an Australian perspective. I was part of the committee which wrote the standard for masonry testing.
Technically, (in at least one respect) the material is the concrete. What you buy the concrete as is up to you. It could be solid or hollow.
As others have said, the blockwork can be reinforced, however, at least here, the blocks which are core-filled / grouted are usually a different shape to those which are left unfilled. The reason being that it provides a more monolithic structural element. And provides better cover for the reinforcement.
Typical hollow masonry units look like this. The one on the top LHS is a “flush ended” block usually used unfilled. The one to its right is an “H pattern” block, typically used reinforced and core filled.
Reinforced masonry blocks are usually made of stronger material than their unreinforced cousins.
In terms of fire resistance, it is usually expected that fire rated blocks will be made with basaltic aggregate.