I'm not fully sure about the Byzantines, but it's interesting that in our relative period (i.e. TGC's times), "Bulgarian" and "Slav" in the Bulgarian sources is one and the same thing - Bulgarian means Slav and Slav means Bulgarian (usually). F.e. when Chernorizets Hrabar refers to the old Bulgar runes, he says that "the Slavs" used to write with them, i.e. for him even "Slav" and "Bulgar" (Proto-Bulgarian) are the same.
Funny though, because the actual Slavs (i.e. South Slavs; as Scolot implied, the early Eastern Slavs were called Anti, while the Western Slavs were called Venedi) were described by Procopius of Caesarea as "being neither blond, nor dark-haired, but rather "reddish" (not sure if "reddish" is the correct translation though, because reddish hair is very rare here - I would've guessed something more like my hair-colour, "dirty blond").
I've mentioned it before (somewhere else though), but the term Scythian, IMO, doesn't necessarily denote an "ethnic" origin, but also a geographical one. Thus, since the Rus lived in the region which the ancients call Scythia (and since the Byzantines loved using ancient names for current peoples), they were also called Scythians. Similarly, the Bulgars were first called Scythians and eventually, as they gained a lasting control over their Balkan lands - Moesians.
Interesting. Which writers did you have in mind and did they refer to the Rus as "Saqaliba" or something else?