bl.uk is one of the few original direct subdomains of .uk that existed before Nominet opened up to direct registrations in 2014. As far as I'm aware, bl.uk, parliament.uk, nls.uk (national library of Scotland) and nhs.uk were the only .uk domains that weren't under .ac.uk, .co.uk, .org.uk, .mod.uk, or .gov.uk before that point.
Probably more like Pope St. Pius V.