Can't remember the exact chemistry now (used it a lot in an old job decades ago), but cold chemi-blacking works by making the outer surface slightly porous (maybe it's a ferrous-phosphate layer? not sure) and thus able to hold a thin film of oil as an anti-corrosion device. It's not as good for this as hot chemi-blacking though. You'll need to keep the part lightly oiled to maintain corrosion resistance (like a shotgun barrel). It's for a dry clutch, so light oil next to clutch surfaces - um.... Apologies for the vague chemistry Edit: A quick chat with Wallwork should be the best info about corrosion resistance and maintaining that...
Cold chemical blacking as far as I am aware is the same process as cold gun blueing.....which is a DIY job. One reasonable manufacturer's product below. https://www.amazon.co.uk/Phillips-PROFESSIONAL-BLUING-REPAIR-MAINTENANCE/dp/B07NFB8LBF/ref=sr_1_18?crid=3T1AFVAZ9NB84&dchild=1&keywords=gun+blue&qid=1576243277&sprefix=gun+blue,aps,160&sr=8-18
As previously suggested, give it a good rub with a wire brush followed up with 10 minutes rubbing down with wet n dry and soapy water, give it a good drying, hang it up and spray on a couple of coats of satin black, it'll come up mint and cost you about 50p. Job done..
