This tool crawled a person's mutuals and cross-referenced their public block lists, surfacing which of your mutuals block each other.
It was built at the request of @mfzx.net, who later asked for it to be taken down — on reflection, a tool that surfaces who's quietly blocking whom inside your own circle is more useful for harassment than curiosity, even when every input is already public. That's a fair call, so it's down.