But, they can't read the message if it's an onion service. Yeah, they may know you're using Tor and that you're also visiting a block explorer, but they can't know what you're viewing. This is only known by the one who runs the onion service. So, unless every Tor node is a honeypot AND the block explorer is ran by surveillance agencies, you're fine.
Every TOR node doesn't need to be, but a few of them could. They could also be getting data from certain block explorers who would gladly sell it to them or hand it over for free depending on their agreement.
Tor obfuscates your entry and exit point to the network. Your exit node can only track you if they are also controlling your entry node and can link your entry and exit traffic between the two nodes, effectively mounting a Sybil attack.
Is this a risk worth thinking about? How many entry and exit node combinations exist on the TOR network? Do the numbers change and increase regularly or are they more or less the same for years?