We do a lot of powdercoating at work (90% black), and the biggest issue for exterior applications, like running boards, is UV stability. Black will fade to gray in 18 - 24 months of exposure with standard polyester powders, regardless of manufacturer. There are more expensive powders (2x-3x price) that can extend that by a year, but don't expect miracles.
Underneath, interior, or trailer queen (no offence intended) that rarely sees the light of day, OK, but exterior of a driver be forewarned.
