You are not wrong to think if it was yours you would redo this weld. This black soot is created by multiples reasons (there is 5 main possibility maybe more), I think in this case is most likely the top tubing was recycled and wasn't properly clean beforehand, but it could simply be he was using the wrong gun angle or had bad or too much ventilation. The weld as good apparent fusion on each side and no apparent porosity (I would grind the surface to be sure but there's probably some), it is not very esthetic looking (not straight) the beads are not equal ether (irregular speed) , If I had to rate it I would probably give it a 5 out of 10 at best. If it be a critical structural weld I would redo but if it is what it look like a simple lock-in backet I would polish and leave it, if there is no porosity showing up by the grind or polish.
The 5 possibility for black soot are;
1) Contamination due to poor gas coverage (poor or inadequate gas flow rate or too much ventilation)
2) Insufficient Cleaning
3) Incorrect Arc length (too long)
4) Incorrect Welding Parameters (amperage or polarity)
5) Inadequate Ventilation (Insufficient ventilation in the welding area can cause the accumulation of smoke and soot particles would create deposite of black soot)