This can have a couple of useful effects given the very limited effort it takes to paint them black. Firstly it reduces the value of citizen intelligence that can be gathered during attacks. For example, drones flying to targets deep inside Ukraine will overfly many villages and homes where civilians will note direction and numbers to pass quickly to defence authorities. Black drones in the night sky make this less useful information. Secondly, Ukraine has attempted to increase the number of cheap gun based defence systems so that low and slow drones can be engaged without expensive missile launches. This is often as simple as spotlights and a heavy MG in the back of a truck. Dark paint could reduce the effectiveness of cheap solutions like that and force engagement from more expensive systems.

Obviously it's pretty hard to say just how useful it will be, but generally reduction in detection increases the chances of slipping through.