it could be that dark due to the tannins coming from the bark where it is trying to heal etc etc...
try higher up the tree and see if that helps the flow....
it should still be ok but a little bitter maybe, it wont kill you thats for sure....
try another tree, use differing levels with a knife tip before you tap it properly...
Just to say to remember that's its still just water that's been through a tree, I may have picked up some nutrients and stuff, but dont expect some sweet elixir, nice yes, if you like tree water