My finger tips get charcoal grey when I play new Ernie Ball Super Slinky’s until they break in. It’s sort of like a graphite film on the tips of my fingers. I just tried a set of NYXLs and that didn’t happen to me.
I am such a freak. I just found this picture from two years ago.
According to PRS, from the website:
PRS Signature Series Strings are nickel-plated steel strings with hex-shaped, high-carbon steel core. They feature a proprietary, reformulated alloy that provides greater magnetic property for louder, brighter tone and soldered ends that extend string life and help the guitar stay in tune.
A proprietary alloy means that the alloy used is exclusive to PRS Signature strings - not used by other strings. They also have Soldered ends too
It does appear that the Signature series though are 'unique' to PRS as they use a Proprietary alloy - even if they are made by D'addario exclusively for PRS