If you get this problem is inside blender too then this issue shouldn’t be caused by Roblox,
try resolving the issue from blender first. However, to try and help resolve the issue…
I would suggest the following:
There might be an issue with the UV placement, check if it’s placed correctly.
There might be a setting that causes it to apply black lines at some edges.
Make sure your workspace is always clear before starting work and
try re inserting the Square mesh in blender [SHIFT + A] THEN add
the UV texture on it ,
I remember people having issue with using the
default given Square shape, so if you’ve used the default square you
might having some settings that you don’t want on it.
I hope this has helped, if not please tell me what you’ve tried and what results did you get.
Good luck Modeling!