Blackstone has 2 textures: 1 for its front, back, and sides; and one for the top and bottom. Gilded Blackstone only uses the side texture, despite being based off of Blackstone. as such, Gilded Blackstone doesn't blend into the top of Blackstone.

There are attached images to visualize the issue.

I noticed it in 1.20.1, but chances are, it's been this way since it got added.

All it needs is a texture for the top/bottom of the Gilded Blackstone to blend in with Blackstone properly.