weight painting prob 2.79

hi all i have a prob when i go to weight paint, the object
is black this happens also with a default scene standard cube extrude and bones added wth weights,
go to weight paint still black object.
can anyone tell me whats going on .it never happened in 2.7

here a shot .Why is this not normal weight painting blender2.79


You must have enabled ‘Show Zero Weights’ on the options panel of the tool shelf, zero-weighted vertices are black.