[gattr]

An interesting usage example for very black materials (which are required to be effective also at grazing incidence angles) is flocking the insides of amateur telescopes. Not that it's always necessary, but can help in some poor designs where the light beam might reflect off some optical tube elements etc. (which results in a decreased overall contrast).

One of the popular choices these days are adhesive sheets of black velour. Another option is thick dark paint mixed with sawdust.