We wanna see pics! Do you know whom the father is? May it be by any chance she the momma is purebred?
Black isnt known to be the most dominat gene, but apparently it is quite strong.
Its not unusual spotted barn cats, for example with the Van pattern or Harlequin black-and white pattern, get the whole litter the same pattern.
I presume it may be so here too, a special case of black-and white coloration, although here its extra everything of black. If you told they were for example, all tuxedos, it would be easier to imagine, more standard.