I've noticed a old few lenses have some type of black substance (residue, coating, or powder) on some of the elements, especially the inner surfaces between the first and second element. The substance isn't visible (therefore, no picture). I only know it's there after wiping the lens and seeing what appears to be a black powder on the cloth.
The lenses appear to be from the 1970-1980s. The first lens I noticed the problem in was a 135/2.8 prime for Canon FD mount. Most recently, I have noticed it on a Tamron SP 28-135/4-4.5 BBAR MC lens (Adaptall mount).
It occurs to me that this stuff may be on the lens intentionally as an antireflective coating. While I hope not, even if it is, it would still be beneficial to clean away because the lenses do improve in wide-open sharpness after cleaning. Stopped down performance is negligibly affected, and I don't notice any particular change in flaring.
Is this black substance there intentionally? If not, what could be getting into the insides of lenses to coat the elements?