I'm no expert, but that straight line doesn't look natural lol. Maybe the way the stone was stacked caused this to happen. Maybe acid washing or pressure washing could help?

I think it is typical of the material, but you don't usually see such a large piece. I am used to seeing paver sized pieces. Your installer should have picked a better piece for that particular location, but not sure what you can or should do now. It will get weathered and dirty and the line should become less obvious.

We deal with a lot of bluestone. I've never seen this. It looks like a piece of Thermal Bluestone not a variegated piece.

Variegated bluestone has natural clefts on the surface. It's not smooth and has many clor variations.

This looks like another stone was sitting on top of that one for an extended period of time outside. It got weathered and that's the result.