Chalk that one up to "Urban Legend" status. I have never seen a negative result that would even be construed as a result of metal in the brush. Yes, if you have a sloppy, rusted brush that is dropping junk everywhere, then you could have an issue, but if you keep the brush clean and in good shape you won't have any problems.

Sometimes people get spots on their prints, and have difficult determining where they come from. I think someone along the line thought that it may somehow be a result of a flake of metal contaminating the solution. Then, looking around the lab, decided that the brush is the most likely culprit.

Rather than the brush, I think it is far more likely that contamination is a result of the heater element in a hair dryer beginning to expel some pieces of metal as it ages.

The most likely source of black spots is the metal salts. Platinum is close to saturation, so some can drop out of that if it gets too cold or some of the H2O evaportes off. Palladium can also get some 'grit' in it. It looks like carbon, but it isn't from a supersaturated condition.

The best thing to do is be very aware of the condition of your metal salts, and filter them through filter paper if they show signs of the 'grit'. If the platinum has crystals in it, you can add a few drops of distilled water and then heat up the solution, and you should be back in business.