Technically, it seems more like the latter interpretation is correct, and since you posted with your PFS sorcerer character I assume it's for said character. So for the purposes of PFS, assume it won't work unless your GM says otherwise.
Personally, given the investment needed and the increased action cost, I'd be fine with adding a fairly weak poison to a spell in a home game.
"You grow fangs" is the effect of Serpentine's power; it wouldn't work. (The other concern I'd have is this: a rounds-per-day mechanic might not be the same thing as a uses-per-day mechanic.)
Also, when you use Sorcerer's Robe on a multitarget / AoE spell, you choose which target is hit by the rider; you don't hit everyone.
...the wearer expends one use of her 1st-level bloodline power and selects one target of the spell to be affected by that bloodline power. If the spell allows a saving throw, a successful save against the spell means the bloodline power deals half damage (if it normally deals hit point damage) or is negated (if it does not). The robe can be used up to three times per day.
