Your texture have too low resolution. Thi’s Roblox pixel blending issue. To fix this, you need do 1 of the following:
Increase image resolution at least by 8 times.
Create 3x3 pixel colors, and then adjust manually every pixel’s on ore UV, so it will occupy middle piece of color on texture.
(Also, I won’t reccommend you use such blocks in high quantities, because big amount of tris will cause lags)
Try re-baking the texture into a new file. The reason for the blurring is because in blender, the texture you use is set to an extremely low resolution.
I’d reccomend watching a tutorial on baking textures in blender if you dont know how.