I assume when you call the cap a "HEI" cap, you are refering to the large 2 piece cap setup used on later Motorcraft Duraspark distributor, rather than the GM style HEI with the integral coil , correct? If so, I can not recall ever seeing such a cap that was black. All I have ever seen was blue, then gray caps and adapters from Ford, and red caps from MSD. The Duraspark distributors as used on the 75 &76 360 and 390 PU trucks were used with the smaller 1 piece caps, I have never seen a factory FE Duraspark that had the large 2 piece cap from the factory. I wonder if there would be any issues if you simply painted a blue or gray cap with black engine enamel?