That's right you must remove the heat tint before passivation. Even with good vacuum heat treatment oven you may get a slightly discolored surface. So, the best is to use mechanical cleaning process such as fine glass bead blasting. However beware of excess metal removal which may affect the part dimensions. Using a chemical etching process to remove the heat tint may result in hydrogen embrittlement and will surely remove too much metal which can be as much as 0.05mm from the surface. The chemical removal process is not accurate and difficult to control the metal removal thickness.

Your best bet is to rough machine the part then heat treat it to H900. Now use fine glass bead to mechanically clean the part and then machine/grind the accurate dimensions of the part to final dimensions. This way you will have a clean part ready for passivation.