The blob isn't a light leak, as it is a low-density area on the film.
If it were a problem of insufficient developer in the tank, you'd probably have low density in several negs.
It's not a shape I'd expect if it were a bubble, but there is faint density that one might associate with uneven development. It could have been caused by strap or some other obstruction either outside the camera or possibly internally. This might be the most likely, as the processing seems ok otherwise.
If this is Kodak, Ilford or Fuji film, the chance of a defect is vanishingly small. That's likely true of the the other suppliers too though perhaps slightly less so. Your odds of winning the lottery are probably much more likely.