Question . i want them with black border and black union jack - is this legal 
Yes. As long as you have the standard font and correct colour it is legal. Border and country badge can be as you like
I've been meaning to ask, my plats have the EU logo one it (ring of stars). Question is if you have that on your number plate, or say the GB flag, do you still need to have one of those oval GB stickers on the boot when driving abroad?
I dont believe you do - which is why i am going for one as i hate those stickers
Correct, as far as I am aware.
It falls down, however, if you have a St Georges/St Andrews or Dragon or similar instead. They are not strictly EU legal but again, from a GB enforcing practitioners viewpoint, not worth bothering about. There are much larger fish to fry.
But I would (and do) put a GB sticker on when on hols, in case an overenthousiastic Gendarme sees the matter differently.
HtH
B