Chicken – My chicken breasts release a lot of fluid when cooked. How to prevent this

I suspect the blackness comes from the spices you put on the chicken burning.

Try this technique I learnt from Jamie Oliver: Season your chicken as normal. Put the pan on a high heat until it's hot (not stupid-hot, just hot). Add olive oil and the chicken skin-side down. Cook for about 5 minutes.

Meanwhile, take a square of baking parchment or greaseproof paper big enough to cover the pan. Fold it in half until you have a 'folded fan' shape. Hold the point so it's roughly over the middle of the pan, and tear off any excess that goes over the side of the pan. Unfold and you should have a circle of paper. Scrunch it up, then run it under a cold tap and give it a shake.

Turn the chicken over, turn the heat to medium low, then place the wet paper (a 'cartouche') on to the chicken. Finally put a heavy pan lid (the lid from a casserole is ideal) on top. Cook for another 20 minutes.

The moisture in the cartouche helps keep the chicken moist and prevents it from burning. Cooking time will depend on the thickness of the breast, so do check it's cooked through before eating (cut it open and check it's not pink).

The only method I have personally found to be reliable for grilling/pan-frying chicken breasts to a relatively uniform doneness is to pound them very, very thin with a mallet or rolling pin. Thin, as in scaloppine-thin, so that it cooks almost instantly in the pan.

Every other stovetop-only method is almost certainly going to produce a bland, tough cut, regardless of whether you press it down or not. As noted in my comment, my usual (lazier) method that does not involve pounding is to get a nice sear in the pan, then jam in a temperature probe and bake it in the oven until it's done (the USDA recommends 165° F, I usually don't go quite that high).

If pressing the meat actually accomplishes anything at all, it would most likely be to just squeeze out whatever tiny amount of precious juices the breast does have, and possibly give you slightly more even cooking on the exterior only; it will not help to cook the interior much faster unless, as stated above, the cut has been pounded extremely thin and flat, at which point it doesn't really matter.

P.S. Salt and olive oil is a terrible "marinade" for any cut of meat, especially a chicken breast. The salt is just going to get suspended in the oil and never reach the meat at all, and the oil itself won't have much of an effect on such a lean cut. You really need to change your marinade as well, preferably to something water-based (or at least not 100% oil).