Question about welding a car?

[Suzz]

So my car had to be welded on the outer lower sill but there was no visible rust and i did not notice until a few days later that there is a horrible black square there now.
Can this be smoothed down and sprayed as it does not look too good.
Just wondering what this black stuff is and why it is there since the welding was under the sill not on the side?
As you can tell i know nothing about welding!
Thanks.