The air bleeds on an Allison 250 should be blanked off during compressor washing. It means that the comp wash will take a little longer, but all that 'crud' that you are seeing coming out of your heating ducts is also getting into the engine's compressor bleed valve air filter (which can effect the bleed valve operation), is clogging up your heating bleed air valve and if it's a Bendix FCU, it's getting into the Pc air filter which could possibly effect your engine fuel governing.
Breathing comp wash fumes won't kill you, but is very unpleasant.....see link for Zok 27 safety data sheet.
ZOK 27