OP, this explorer could be a good addition for your research,
https://explorer.crystalblockchain.com/Bitcoin is fungible. Like cash, whatever it was used for before you owned it doesn't matter.
The idea of "taint" goes against everything Bitcoin stands for. It's something we need to fight for Bitcoins existence.
Although "taint" from the network's viewpoint DOES NOT exist, I believe such arguments in your post are in denial of what could actually happen to you in practice. Try making a deposit in Coinbase with UTXOs you received from a Dark Market user. Therefore "taint" from the real world's viewpoint is real, and because Bitcoin is easy to track, each Satoshi has a "history" and a "context" because any entity can put "taint" labels in them.