I got one of the recent Aim Surplus european ex-police BHPs - it is a Mk2. At first I assumed the dull matt finish to be parkerizing, but looking at the color more closely it seems to have a slight blue/brown tinge to it. Could it be hot blued instead ? The worn areas are polished smooth (holster wear), so there is no evidence of bead blasting. Some areas of wear have a sharp transition (almost like chipped paint) while other areas are faded. I'd like to know definitively which finish I have before I start restoration of the pistol. Here is a picture:

My 1989 mark ii is black paint over a parkerized base.

The overwhelming majority of MkII HP's were parkerized. Some were dark gray/black and others a grayish green. The color difference is a result of changes to the chemicals used in the process, as each batch of parts gets its turn in the park tank the color will continue to change slightly until refreshed.
I would bet your HP was orginally parkerized.

take a tiny amount of naval jelly or other rust remover next to a spot that is already 'worn thru'.

if its blued (a form of rust) the finish will come off

That HP looks like the ones that TGI Imported a little while back.  Who's import mark is on it?
I thought these pistols were "professionally" refinished.