AFAIK, i think roger worked with rich cocco to develop his string line, but the sadowsky strings sound quite different. i've owned slappers for a VERY long time, and the sadowskys have a nicer upper end air and detail, and the slappers had more midrange chew. the sadowsky blue labels are similar, but are smoother feeling, with a much nicer detail. also, the slappers died on me much faster than the sadowskys.
the sadowsky black labels are a bit more scooped sounding vs. the blue label. admittedly, when roger was first testing nickel strings, i remember he gave me two sets to play around with. he ultimately went w/ the string that i actually didnt quite gel to, which became the black label. while they are awesome for slap, and i have them on one of my beater 4 string practice basses, for my particular fingerstyle, the black label were just too smooth and scooped in the mids for me. but when the blues came out, they instantly reminded me of that particular test string set that i really liked!!!
also, i found it easier to darken a brighter sounding string than it is to bring life or detail out of a more darker sounding string. most times when i dial in more high end for articulation, i get too much hiss and noise for me. but darkening a bright string doesnt introduce extraneous noise. rather the opposite.
thusly, at first, i thought i'd never need the VTC, but after a while, realized its terrific in dialing out juuuuust enough upper end bite for a smoother sound when i switch to pickstyle.