I actually think Blake would be a horrible backup for us, but a potentially decent starter for another team, or for us if we get a playmaker/point small forward.
Look at our White Team. Let's say that Sergio's gone, and it's some combination of Blake/Rudy/Webster/Outlaw/Przy. You don't need another shooter on that squad...you need a distributor/traffic cop. A decent defender would be nice, but I'm not reaching for the moon here. Blake is neither of those.
Steve Blake is the 3rd-best SG on this team (maybe 4th if Webs is healthy--but that's another debate). STOMP made an amazing point that Kerr was able to play PG b/c Scottie was the de facto PG from the 3.
Blake, imo, could play the role (I think) we all want him to play...for about 1/3 of the HOU series he was aggressive...hitting the open 3 while having a moment in Game 3 (iirc) where he had 3 assists for P&R dunks in a row--and then never went back to it. He's well below-average in leading the break.