I got my bag of sand in the mail today. (never thought I would put those words together in combination haha)
Honestly, I think it'll do. It's a bit more dull than the coal slag but I do believe this is a case where having ALL sand will make that dullness less apparent. I also think the people reading this thread are the only people that would ever even notice because they're aware. I'm also open to sacrificing a little bit of realistic appearance for the operational realism. I had a good time by myself shuffling hoppers around and unloading them to test how the sand flows.
The sand seems to catch a bit of static electricity or something. It's got a bit more of a tendency to stick to the rails than the coal slag. It clumps up in the hoppers like the coal slag though. I also noticed more fines in the sand but that's probably because I emptied the bag. I never made it more than three pounds into the bag of coal slag.
I suppose I'm not too concerned about the sand on the rails because I had already intended to remove some material in to reduce material even getting stuck anyway.
I'm thinking this is the solution I'm going to go with. It seems to have less dust than the coal slag as well (just has some fines in it). Hopefully it stands the test of time. I don't know if it's dyed black or actually black. It's not as cost effective as the coal slag but I guess I'll live. I'll have to spent some more time emptying it to see if it escapes the hoppers or sticks to plastic too much but this definitely looks promising.
Thank you all for the help! This thread is going to serve some people will in the future.