I am 10 year old, started my period 3 month ago and this months period I have been bleeding out BLACK blood. What does this mean?

I don't think it is "black" blood that you are seeing... rather heavily clotted blood that's looks deep dark red, and somewhat appears to be almost black.

This is normal, as it is probably just a heavy period and your blood is clotting.

It is also normal for it to be brown, as that may happen too just to warn you. This is also your blood just simply being exposed to air and is clotting and turning brown.