Material alpha blending causing black holes issue

hi there,

i’ve a subdivided plane from max and want to apply a two way blend which i found in the documentary from ue4 itself. i set the material and want to paint with the alpha the 2nd texture to blen both as you can see on the following picute. but when i start painting i just getting these black holes and i dont know why. when i paint the other texture back then it turns normal again.

what’s the resaon for this issue?