As stated earlier, rust blueing is probably the best all around process for a deep blue that will last for years.. Cold blueing can be enhanced by heating the parts prior to application of the blue solution. What I have found that works to help set the cold blue finish is to dunk the parts in oil and let stand untouched for at least 24 hours. This has shown to do two things: Sets the finish and makes it darken.
I typically use WD-40 for this process and then wipe clean before applying a perminant coating of a high quality gun oil. I've got several firearms in my cabinet that were done over the past several years and they still are as nice as the day I completed them.
But remember this most importantly... PREPWORK is EVERYTHING! I always wipe down with acetone prior to blueing.. It's amazing how dark those paper towels get off what appears to be clean steel.