Ratchet

Thanks.

I choosen Maya option for object and texture export and seems are no more visible.

Also i found a tutorial showing a way to fix seams by using a smear brush and smear around seams on the normal map texture.
I did not use that, but could be usefull if the issue would appear.

Ok found it.
Click on top righ camera icon -> Background -> Vertical Gradient
Really hard to find while could have been lot more intuitive , for example on menu view -> clear background image