Hi what do you wanna know about them? Are you trying to ask if your swordtail is pregnant?
Usually the black thing that you we're talking about is called Gravid Spot only females have those and sometimes when they're pregnant they get really really black and gets bigger and one more thing they also get really huge like about to explode. Actually i got 1 Swordtail female at my 10 gallon tank i just separated her today because she has sign of giving birth around this week. not very soon. But it will keep her calm and not stressed before she gives birth. and also one of my swordtail female just gave birth on my Community tank good thing i have a little bit of floating and planted plants so the fry can hide.
Signs:
1. Lack of Appetite
2. Will gonna get a food like flakes then she will spit it out crushed and blended.
3. Always wanna be alone or always hide usually on plants, back of the heater, or filter hose.
4. She sits on the ground feeling really heavy.
Some swords doesn't really show signs they just pop it out like somewhere no one bugs him. Idk my potential preggy swordtail right now is acting very weird. I think she hates me.