It is quite true that Black powder is quite capable of building excess pressure. However, the vast majority of cartridges designed for black were designed for all the powder the case would hold up to the bullets base, usually compressed somewhat. Normally there simply is no place to put an overload. In fact even with old style baloon head cases most find it difficult to get the amount the cartridge's nomenclature would indicate. Did you ever try to drop 70 grains of black into a .45-70 case & then try to seat a bullet for instance??

Miller/TN
I Didn't Say Everything I Said, Yogi Berra