you sure they aren't using "fluorescent" mono (like "clear/blu")
I wouldn't think "flurocarbon" line would have any "fluorescent" properties in it .... if it did, it would just be another fluorescent mono line (minus the stretchiness).
And - yes ... fluorescent mono (clear/blu, gold, yellow, pink, or whatever color) will look like it's four times as big ... and will "light up", under a Blacklight. Other (white) lights in the vicinity will reduce that somewhat, depending on how bright they are. Blacklights will also cause the "fluorescent paint/tape" on bobbers & rod tips, to "glow" as well !! But, you won't get much effect from a $20 Blacklight, except on the rod/line that is right next to it. They don't have the range of the ones with 12" bulbs (at least not in my experience) .....cp