Hello Andy,
I have only used the semi-hot bluing salts - its difficult to mask with these and you have to buff away the blue from areas you want to appear bright. This type of blue is very tenacious and hard wearing compared to the cold-applied bluing mixes, so if you have pits in areas that are to remain bright - no matter how small they are, they will have to be levelled to get the blue out.
A safer option would probably be one of the cold blue mixes you can get from gun shops - these don't adhere particularly well, and you can mask areas (with nail varnish or rubber cement for example, after thorough degreasing). You would have the same problem with the scales rubbing the blue away again, so it would be better to dismantle and put washers in place to keep the tang away from the scales.
Otherwise just buff all the blue away and leave bright!
Regards,
Neil
PS: just occurred to me - you could remove the blade, remove the old finish, degrease and use the old method of rust-bluing. The bits that aren't to be blued are coated with wax or bitumen (etchers hard ground). I won't go into the process but it is very simple and there are a lot of instructions on the web. Takes a bit of time, but is said to give a superior finish with a plummy red/blue finish.