Do I have to let my Dahlias go black before cutting them back.
I leave most of my Dahlias in the ground over winter but cover them with fleece if a frost is forecast.
These are not very often nowadays living here on the Isle of Wight.
They do continue growing to quite late in the year but not many flowers.
I would like to cut them back now in November and not wait for the first frost.

I have read many times as long as they are in the ground for at least 120 days you can cut them down and dig before frost.
Since you aren't digging I don't see why not. I am no expert though that's for sure Hopefully, someone with more experience will chime in.

Day length is most important... over 14 hours of light they grow only feeder roots and vegetation, less than 10 they only fatten tubers, gradually moving from one phase to the other in the middle ground.
I read this in a published agricultural college research paper about rooting cuttings and it appears to be the case in my experience, so I try to keep them in the ground for as much photosynthesis as possible during sunny short days. I will protect them if I see one or two cold nights that are predicted to be followed by a week of mild/warm weather, otherwise I'll let them get hit with a frost and dig them up 3-10 days later.
I keep that window because I've heard cold ground helps eyes develop, making dividing before storage easier, but I've been unable to substantiate that piece of wisdom with any published research.