I've had good luck with Oxpho cold bluing cream, but there are tricks to getting it to work really well.
1, You have to be EXTREMELY clean. All your metal surfaces have to be 100% grease & oil free. Wear gloves & change them very often. I've gone as far as submerging parts into straight acetone & leaving it 24 hours in preparation.
2, Get your metal good & hot before you start. A heat gun is perfect. You want the solution to sizzle when it hits the metal. Re-heat once it's applied.
3, Wipe on the solution, don't rub it! If you rub, you'll get a coppery sheen that you can't get rid of.
It's always going to look like cold blue, is the other thing. There's a noticeable difference in finish, compared to rust or hot dip.