Well there is POWER[0], not that it's a very affordable choice, the cheapest motherboard + cpu costing $1,732.07

[0]https://www.raptorcs.com/content/BK1B01/intro.html

Is ARM an option? I know in practice many ARM systems rely on blobs, but it's not clear to me that that's universal.

There is nothing like "ARM" in the market.

There is CPU/SoC X by Y, built on top of a license from ARM.

So you would have to boycott N companies instead of just AMD & Intel :( And even more so, since all their customers are EOM that are happy to sign NDAs.

Decent blob scarce arm option: https://libre.computer/

There are ARM systems that don't need blobs. Whether they are useful to you is a different question. An example is the iMX line from NXP.