The Bitcoin blockchain stores data about bitcoin users. This includes:
- There bitcoin address, which is linkable to their identity via blockchain analysis
- Transactions they have made
This would seem to constitute personal data. Would a bitcoin node need consent from any EU citizen who has data on the blockchain? What if an EU citizen made a GDPR request to have their data removed from the blockchain? What about other cryptocurrencies, like Monero, that store transactions but not monero addresses or amounts?
Note that almost any cryptocurrency blockchain can also store arbitrary data (in case the above would not count), although this would be considered a "misuse" of the blockchain in most cases. (There are some blockchains that are designed for arbitrary data storage, however.)