There are many 'wedges' of RAM in the leading edges of the SR-71 wing. Others on other parts of the airframe also. Even on early A-12's (which were mostly left natural titanium colour) these areas were always black. This made it harder for a casual observer to spot the wedges. The contrast twixt the RAM & the natural metal would have been obvious. These aircraft though were basically silver, so the black is plainly not significant in any aerodynamic or heat dissipating characteristic that the a/c has. It's for tactical reasons and to enhance 'stealth' ie reduction of radar cross section. I believe the paint (which may have Iron ferrites in it) helps to attenuate and dissipate radar energy that strikes it.