Depending exactly where you live, you may well have experienced some extended warm and dry weather recently. Since rhubarb likes it cool and damp, this is likely to have affected output. Also it will have used up nutrients in the soil. In general the oxalate content of the stalks goes up later in the season too, and the plant needs some energy for next year, so usually people stop pulling in high summer to give the plant chance to recover, and not eat too much oxalate.

Black powdery stuff sounds like mould. Rhubarb stalks that are not pulled will go rotten after a few weeks. Be more worried if this is occuring on fresh stalks.