How to make a dynamicly revealing minimap

Im making a strategy game and Ive been fighting to make a dynamicly revealing minimap, cant make it work.

Its a material that takes a bleding of two maps that are beeing captured by a capture-camera Blueprint every x-frame and projected onto a black plane. Pawns spawn planes with a radial gradient ramp to make a white area around the black plane and the capture-camera BP captures that and every short while planes are beeing cleared for memory purpose. And becouse of that theres 2 maps to add the detail to each other in turn. But the problem is with the Material in which two maps are blended, the image from the plane gets darker and darker and over time gets washed away but if I increase the intencity, for some reason it does something like an “explosion of the exposure” effect.

Is there any easier way to make a gradualy revealing minimap, just like in this video :

I’ll post my video showing what’s the problem in a bit.