mutuals of X · who block Y
Give an account X and a third party Y. This finds X's mutuals — the accounts X follows who follow back — and shows which of them block Y. Mutuals are computed from public follow records (X's follows, intersected with who follows X back); the blocks are public app.bsky.graph.block records on each mutual's own PDS. Read straight off the network: no login, nothing written. Flip the scope to all followers to widen the net to everyone who follows X, not just mutuals — a deeper sweep, slower for big accounts. For an arbitrary set of accounts vs a set of posters, use the block matrix.
Public data only. Direct blocks only — a "no" here can't see blocks applied via a subscribed moderation block-list (those write no per-account record). This walks each candidate's block list, so a big mutual set — and especially the all followers scope on a large account — can take a while; progress is shown below.