Try making a deposit in Coinbase with UTXOs you received from a Dark Market user.
Solution: Don't use Coinbase! Aside from being anti-bitcoin, anti-privacy, anti-fungibility, and pro-censorship, they are deeply unethical, support attacks on bitcoin, sell your data, report you to your government, work with a variety of government agencies, insider trade against you, employ literal human rights' abusers, and will arbitrarily freeze your account and seize your coins with no warning and no recourse. Why anyone would voluntarily use them is far beyond me.
Therefore "taint" from the real world's viewpoint is real
Taint from Coinbase's view is real. Don't use Coinbase and whatever they classify as tainted does not matter to you in the slightest. I have a bunch of coins which I'm sure various centralized exchanges would class as tainted, but I don't give it a moment's thought and it doesn't affect me in the slightest because I choose not to support such anti-bitcoin and anti-freedom businesses. There are more alternatives available than ever before.