As someone who uses private trackers, bear with me for a moment:
On private trackers, your download and upload is tracked. You are required to maintain a certain ratio.
Most tracker sites allow users to "donate" money to get extra ratio.
Most tracker sites allow users to request new files and pay a bounty, in uploads (i.e. ratio).
In effect, it is already a currency.
Trackers get seized from time to time, and if the entire state of a tracker is stored in a decentralised manner, then seizure is no longer a risk.
The dataset stored by a tracker, in the age of magnet links, is very small, and can be replicated amongst all nodes.
So, as someone who usually rolls my eyes at people saying "blockchain", this is actually a perfect use for blockchain technology.