Hi Ken, is this a hot salt or a belgum blue? it sounds like belgum blue.(thinking while typing).
could be humid enough before bluing starts to rust the metal a bit, oil or grease that the cleaner did not remove, residue left over from solvent to degrease parts, it sounds like a contamination of sorts somewhere in the prosses.
What color are the blotches?
Ken, call anytime tomorrow. If i can help i will, Jim Clark is our bluing guy when it comes to belgian, he's been doing this for 30 years and has been through all sorts of stuff with it.
The shop number is 913-782-3131 ext. 101 for the front counter guy or direct to the shop phone ext. 103. we start at 0700 kansas time, it's snowing now so jim might be a bit late, he drives a sissy jeep.
I would likely suggest to anyone having belgian blue troubles to call the guy selling the mix to Brownell's for the last umpteen years, Art at Art's Gun & Sport, 636-944-3630. 9-5 CST
He makes the stuff, he sells the stuff, he uses the stuff. He's even got a website now.