Four String Set tunes to G-D-A-E. Strings also sold separately.
Black varnish protects the strings from hand perspiration and oils. It extends the life of these gut strings, so you delay that "de-laminating" or "furry gut string" effect.
The sound is remarkably similar to the plain gut strings, so you can mix-n-match. The sound is more direct than the old Golden Tone strings that were gut core, wound with nylon.