Technically, the asteroids are black because of their very rough surfaces, which scatter and absorb all light--although the white outlines wouldn't make much sense anyway. It's a stylistic approach, really.
The weapon isn't a laser or a particle emitter per se
; it's a hybrid weapon that generates a variable-profile beam capable of carving large areas in one quick shot. Here, it's cutting a square-shaped outline into the hapless rock that got in its way. Compare this to a traditional energy shaft: narrow, often over-powered, and requires careful control for longer periods.
Thanks for the comments, as they do help me think more next time. See you around!