Nazeing produced a full 'Bristol Blue', and a slightly less rich blue, again in a solid colour (might have been for Boots the chemist) - but the blue here doesn't immediately suggest Nazeing - it lacks similarity with the shade of blue and the bubble structure for Nazeing. Might be wise to hold off a full confirmation until we can find an example for comparison.
Somewhere on the Board, in recent times, I was looking at a post from Nigel Benson in which he provided images of Nazeing pieces that don't occur in Geoff. Timberlake's book, but par for the course I can't now find it.
Any chance this might be Alum Bay - has some similarity with their blue and streakiness.? Just a thought.