Was the drum moving when the blob appeared, or was it stopped? If the clock mechanism had stopped, I could see why there was a blob. However, if it was moving, as in the visible trace in the photo, I would ask what is different between when the blob formed and when it did not? Other than a stalled or erratic clock mechanism, it could be a "bubble" in the paper, where it was warped or curled, some defect in the paper itself, or one of the problems mentioned in the article at: "The two halves of the nib bucket should be of even length..." If only one point is touching the paper, I think you might get a blob.
Dunno. I was wrong about the paper needing to be absorbent (see above Metcheck URL). I suggest you try a variety of paper types and see which works best. Coated, not coated, 22 lb, vellum, photo paper (glossy or matt), butcher paper, wax paper, premium clay coated plotter paper, etc. Visit an art store and see what they have to offer. Also look for "pen plotter paper": The pen is critical because most "plotter paper" is for inkjet plotters.