The clyinder sounds like a different alloy and cold blue is a plating process.
Cold blue chemically plate (false plate) copper sulfate on the metal and the acid in the solution blackens the plating.
Hot blue would not turn the clyinder black very well and neither will come blue.
BTW bluing is a rust, nothing more.
As for the oil migration, you are suppose to completely disassemble for any finish and before clean the metal.
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