Ok the couch gag is made up.
Where to start? I guess the first thing to mention is that fluids like blood and semen DO NOT FULLY COME OUT IN THE WASH. So how come nothing showed up after the broad's mom supposedly washed it? Retarded.
From there we can move to the stupidest part of the story: By this story's account, the mom WASHED the couch before it was given to the roomies, yet they used LAUNDRY DETERGENT to fool her into thinking there was cum on the couch? Let that whole thought stew in your mind for a second...yeah.
Furthermore, if laundry detergent glowed under black light then every single hotel anyone has "swept" for stains, would glow brighter than baby jesus' fucking aura. Same thing goes for clubs that only use black light; everyone in there would be glowing.
Bottom Line: once something has blood/semen/etc. on it, no amount of washing will ever make it NOT appear under a black light no matter how clean it looks. Also, laundry detergent does not glow under black light. That is so counter intuitive it makes my head spin.