My parents have British passports issued in the early ‘80s (expired, obviously, with the corner cut), and those passports are black, quite plainly and obviously

My parents have British passports issued in the early ‘80s (expired, obviously, with the corner cut), and those passports are black, quite plainly and obviously. The new blue colour sometimes being issued is completely unrelated to passports issued before the standard red EU ones (and is indeed closer to the currently-issued EU Croatian blue passports).