it is very hard to tell what it could be, but could it be a repair? Maybe the wall had a deep groove or a hairline crack which they fixed by adding some material (welding) then honed smooth?
Honestly I do not know what it is and dont want to freak you out, but it is weird
I am in the Bronx. I can take a look at your car and tell you what needs fixing. If you would like to do so, pm me and we will arrange a time and place
Is it possible to modify the rear cover instead of turning the rotor? Can you not grind some of the bracket off? By the way, Can you turn drilled rotors?
usually, blue smoke means burning oil. how many miles on the engine? you can do a leak down and/or a compression test to see if you have bad rings, while you're at it, put new plugs in and see if it helps