Lets expand our thinking here. While everyone is focussing on where the black is coming from, perhaps it comes from nowhere ! Meaning it may come from the filter itself. As we know the octane at the pump is a cocktail of compounds and the fabric the sock is made of reacts with one or more of these compounds. A simple chemical reaction !
If someone has the patience, time and money, take a new sock and let it sit in a clear glass jar full of fuel. The fuel needs to go through cycles of cold and hot (since it sits on top of the tunnel, and behind the engine when on the sled). For how long you say ? That's what the experiment would determine. Just to make it more fun, you'd also have to consider the geological sources of your fuel, because that will affect its composition. Lastly, pressure. We need to increase pressure to match the pressure found in the gas tank of your sled.
Then again, maybe BRP could change the polymer the sock if made of to a more inert material. Or simply figure out a solution so their customers don't have to, and issue a recall. Uhm...what a concept !