is that an inking error?
I cannot tell the condition of the note in question but I suspect it is not in UNC or high grade. This note looks more like soiling.
Most ink spot errors look like spots or blobs of ink. Take a look in Charlton for some good examples.
The other possibility is that there was jamming during printing and the paper was stuck in a roller, and there was residual ink (or dirt, oil, etc) that smeared on the paper. This won't really matter as most collectors won't be interested in this kind of note even if it's an ink error. It would need to be more dramatic to be valued.