You have two options:
1) Say nothing, hope the parent doesn`t bring it up again and then tread on eggshells for awhile till everyone relaxes or
2) Come right out and tell the mom that you felt bad about the bruise and spoke with some experienced providers during naptime and was told that it is very common for a toddler to get a bruise on the bottom from little falls if they land on a block or whatever. Then you be able to guage if the parent sort of passes it off or wants to keep going on about it. It also lets the parent know that you don't hold it against them cause it could happen at home on a toy too.

I am assuming the bruise is more the top of the leg where the elastic is supposed to be and not right across the bottom which would take a pretty big wallop to make any kind of bruise through a diaper be it block, hand or other.