Sudden black fly infestation!! HELP
Time to look into every nook and cranny.  Pull the fridge out, check all cupboards, clean under all furniture.  Location can be anywhere were there is a food source, which can be just about anywhere food or organic material can fall/kicked under/forgotten about.  

Start with the obvious locations first; in/around trashcans, pantries, fruit/vegetable storage, fridge and cooking areas.  If you have pets, check any area where they could possibly get and leave a mess (under/behind furniture).  

Also they have a life cycle that is anywhere from 1 week to 4 weeks.  If you kill the first wave, keep up looking/cleaning as the second wave can still be in the process of maturing.