Foxwoods has a cheque counterfeiting problem and those filthy dirty black chips they've been using all these years have been replaced. They're no longer taking the old ones at the table. They are cashing them at the cage, but who knows for how long? Anybody who is holding black from there had best take care of this quickly.
I saw a PC checking a rack of black chips with a long-wave UV light. I don't know they were looking for.
There actually are some places where the chips have UV markings-- can't remember where, but I've had at least one place in Vegas put my purple chip under a UV light when cashing it-- there were glowing dots on it when they did.
Of course, they may just be curious about what people have been doing to their chips, as Rhino suggested....