I've used the Black SR fuses in most of my electronics for a year or two.
They aren't either a gift or snake oil, but somewhere in between.
Some sound improvement, but with 6 fuses in my amp, it's almost a grand to outfit it...not sure that the improvement was worth a grand in retrospect.
My amp blew a fuse...which one(s)?...I don't know...I can't see the filament in the SR fuses.
Wasn't willing to swap one fuse in and out to troubleshoot as I have to disconnect cables, pull the amp from the rack, take the bottom & top of my amp off to do that...and doing that 6 times or more, right...
So, I just replaced them all with clear glass fuses. The sound? Not as "organic" as with the SR fuses, perhaps, but not all that different.
I like the sound improvement of SR fuses, but the usability/practicality leaves a bit to be desired.
I'll leave the SR fuses in the other equipment, but I won't be buying any more of them.