The most likely reason your AC filters are turning black is because they are not being often changed enough. Over time, the filter collects dust and dirt from the air, which turns it black. If you don't change your filter regularly, the dirt and dust will build up and eventually cause your AC to stop working properly.
Another possibility is that the black filter is due to a problem with the AC itself. If the AC is not properly ventilated, the filters can get clogged with soot and turn black. This is a serious problem that should be fixed by a professional.
If your AC filters are turning black, it's important to figure out the cause so you can take the necessary steps to fix it. Ignoring the problem will only make it worse and could eventually lead to costly repairs.