Tannin used in the tanning process, plus iron, makes blue / black spots. (Check out the vinegaroon process used to produce black leather without using actual dyes.) Looks like there may be some contamination with iron filings, maybe from one of the tannery's cutting or finishing machines. Dampening the leather will likely make the spots larger.
In your own shop and work areas, try to make sure that no iron filings get transferred to your veg tan leather.
Tom