Swordtails do come in black. They are pretty easy to care for. I would try to get them an aquarium as soon as you can. I'm worried more about the water ph level. A filter is a definite in order to keep the water clean. It is never a good idea to change their water all at once as it can be a drastic shock to their system. Only feed them what they will eat within a few minutes as any food left over could dirty the water quickly.
They are very cute by the way. It has been a while since I've had an aquarium, I miss it. We used to have 3 set up and running at one time.