It sounds to me like it could be mites, especially since you mention that blacky has been overgrooming and scratching at himself. The way you describe the bugs is spot on with how mites tend to look as well. One of my old lizard's wood substrate got too damp and I had an issue with mites with him. I ended up throwing all his old substrate away and buying repti-carpet but for hamsters this isn't an option. How many exactly have you spotted? Lift substrate out in your hand and see just how many there are. The sock would be a good idea for checking
If you see a lot, though it'd be stressful, I would throw away all bedding and wooden things and replace them, maybe shoot for a paper based bedding if you can? Purchase new wooden toys and avoid contact with too much moisture. Mites can be introduced via bedding so what could be happening is that they're being brought in with whatever bedding you have. You could try getting your bedding from a different source and/or freezing the new bedding for 24-48 hours. Hope the issue resolves soon. Best wishes