Sometimes it is difficult to disperse black pigments on polymers, resulting in agglomerates or clumps of pigment particles. These can make for places where cracking can occur when the plastic is stressed. This could be worse in the area of the ultrasonic staking, since the polymer is likely somewhat weakened by the welding process.
It is probably more likely that the black pigments could be increasing the thermal properties of the plastic, so that the material is heating faster causing more degradation than the other colored parts. It may be necessary to adjust the ultrasonic staking conditions for black parts with slightly lower amplitude, less energy input or shorter duration.
Finally, there can be problems with the way the color is added. Assuming that they are adding a color concentrate at the molding machine, if they are adding too much or maybe adding a concentrate that has an incompatible carrier polymer, like polyethylene or nylon, they may not fully blend in the plastic, creating contaminated layers of material.