The word is in itself svart and already ends on a t so it stays svart even in the ett-form. If you would replace it by another colour like grön or gul you would use the en-forms, because dörr is an en-word.
I could see that in some cases. If you were asked something like "is the door several colors?", then "only black" would be appropriate, but if you were asked "what color is the door", then "entirely black" would sound more natural.
ganska in Swedish means 'pretty', 'fairly', as in 'to some degree', so that's much weaker than entirely. (we sometimes translate it as quite, but that word is ambiguous in English and can mean either like 'totally' or 'somewhat').