"Although the kittens are black it doesn't mean the mother is necessarily the same colour. They can have all sorts."
This. The litter our kittens came from was comprised of one black, one beige and two ginger tabbies. Their mother was a (gorgeous) tortie/tabby and their grandmother was white with ginger splashes outlined with black. We have the black and the beige ones, who're about as opposite to one another as it's possible to get - and definitely have Siamese in them somewhere!
Also, not all kittens are born with blue eyes. Some are, yes, just like some babies are... but not all. Our black kitten, at one week old when we first saw (fell in love with) him had black eyes - they've now lightened into light brown/hazel. His sister still has bright blue eyes with a hint of green to them at 16 weeks. Their predecessor, my late great Tuxedo cat, was fostered (by me) at 3 weeks old, having been separated from his mother somehow, and his eyes were green/yellow and stayed that way until he left my life 22 years later.
Hoping that mum wasn't the black cat you saw on the road, OP, especially as your trio of trouble look like they might have tortie in them (and I might be wrong, but I'm pretty sure I remember being told that only Queens can be torts...?). Hopefully their mum was spooked by fireworks and didn't return to your shed because of that, and that she'll reappear soon.
They do look around the same age as my Tuxedo did when I fostered him. Their profiles haven't "popped" just yet, and they have that wide-eyed and clueless look about them (which is very sweet). If they're anything like our troublemaker and diva, though, they'll cause chaos whilst looking like butter wouldn't melt... 