The amount of black will vary with the individual fish, especially with a variety like the yellow one. In the wild coloured variety, the black area is much bigger than in my fish.
With gouramis, the male will build a bubble nest in area where the flow from the filter is weakest - water flow scatters the bubbles. When he's satisfied, he'll start courting the female by swimming vertically in front of her with his nose pointing at the surface. She'll ignore him at first so he'll chase her, he'll probably nip at her tail but that will heal quickly if the water is clean. Eventually, she'll go under the nest with him, they'll wrap around each other and a few eggs will be released. For a couple of seconds, they'll both look like they're dead, quite alarming the first time you see it. Then one of them will come out of the stupor and start sucking up the eggs and spitting them into the bubbles. Some males will chase the female off, some will let her help. They'll wrap over and over again till there are lots of eggs in the nest. The male will then chase all other fish away, but if there are any other fish in the tank, they'll eat the eggs. He can't guard the bubble nest while he's chasing a fish, so the others will spy their chance.
In the past I've tried removing the eggs and putting them in a fry trap. They did hatch, but the fry didn't live long. From what I read, they need careful treatment to develop and grow. So now I just leave the eggs in the nest, and they rarely last overnight. Besides, I don't have anywhere to put that many baby fish. If you do move the eggs, they'll hatch in a day or so to a non-swimming form when they feed off the egg sac. It's about a week after laying that they become free swimming, and that's when they need to be fed. You'd need something to feed them on then or they'll starve. Very tiny food, as the fry are very small! If you move the nest and the male into another tank, he'll keep putting the eggs back in the nest when they fall out, and he'll do the same with the non-swimming fry. But as soon as they become free swimming, he'll eat them so you must remove him at that stage. I've never tried it, but I've read that you can move the nest by sliding a piece of glass very carefully under it.
Female platies will have fry roughly every month. I know it's very tempting to try to save every single one, but you'll have to find a home for the fry - and even if a shop will take them, they won't take small ones, only once they reach the same size as the ones they are selling. It sounds very cruel, but the best thing to do with the fry is nothing. If you have a lot of plants (real or synthetic) some fry will survive being eaten, and that will probably be more than you can deal with.
Edit: I must learn to type properly
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