Hornblende is black and, like all amphibole minerals, is characterized by two perfect cleavages that intersect at about 60° or 120°. Instead of breaking in broad, single cleavage planes like the micas, the cleavage surface of an amphibole mineral tends to display a large number of small, offset planes that help give the mineral a splintery appearance.
The large number of small offset cleavage planes may make it tough to spot the angles at which the two cleavages intersect. The solution is to slowly rotate the mineral in your hand and spot the flashes of reflected light. Even if offset, multiple parallel cleavage surfaces will reflect the light at one time. If you keep track of how far you rotate a given sample, you can estimate whether the two sets of flashes occur at 90° or 60°/120° from each other.