Yes, the uneven surface of a brick coupled with matte black BBQ/high thermal paint
is ideal for that.
For even less light escaping from the side (180 degree emission from the bricks face) you should consider using a dark pipe with your brick at the end of said pipe, and shinning the laser through the pipe before it gets terminated on the brick.
This is good for higher visability/power, but at 100mw of 445nm, its very little to worry about.
That will work. What GBD mentioned will also make it safer. I made my beam stop by spray painting steel wool with BBQ paint and then stuffing it into a pipe that is also spray painted. The steel wool works extremely well.