Technically the unit of measurement is hashing power, rather than % of nodes or participants, but there is an underlying 51% problem with blockchain.

The attack surface is similarly limited; they couldn't really "rob" the wealthiest wallets, but they could prevent their transactions from ever happening. They could also spend cryptocurrency they don't really have. Here's an intro:

https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_...