//I seem to recall during the Brexit debate, the two most important things to Brexiteers were the number on the side of a red buss and blue passports.//
I think they were pulling your plonker.
That said, I'm looking forward to receiving my new one when I have to renew later this year. Not because it will be blue or black, but because it will not have the words "European Union" embossed on its cover. That organisation is not empowered to issue passports and quite why it sees the need to have its title printed on the cover of those issued by sovereign nations (and why those issuing nations allow it) is something of a mystery.
I still have my old pre-EEC hardback passport (with the corner of the front cover snipped off). It is a very, very dark blue.