My information is from Robert C. Renneberg, "The Winchester Model 94", (1991).
1.) 500,000 to 1,200,00 (1909-1942): Recievers have a high nickle content and the bluing often "flakes" off, even when the rifle is in mint condition.
2.) 2,600,000 and above (Post '64): Reciever is an investment casting made from some unknown alloy which does not accept hot bluing by itself. They are plated with iron so that they can be blued.
It sounds like the majority of Model 94's do not hot blue well at all, but I have no practical experience in this area.