It may be the piano, not you. If your piano is not maintained properly, assuming it is an excellent piano, then many things will suffer, most notably, the pianist.
If it's not the piano, the it must be the pianist. There are different kinds of touch that will produce different sounds. From what you describe - losing control, slipping - this may very well be the case. The touch to play pianissimo is different from playing piano, which is different from playing loudly.
By touch, I do not mean the force exerted, but how it is exerted.
Out of curiosity, what technique do you use to play scales? Do your fingers curl when playing the black keys as you would the whites? Knowing this might help.