RealGM

How good was he as a scorer? He wasn't shooting that much threes back in the day, would he easily adapt to today's game? How was he on defense? Where would you put him among all time international SGs?

First off, not sure if you can really count him as an international. He lived in America from a young age, and played for the USA (or at least, was named in the team).

He was a good scorer, and a tough cover. Got his jumpshot off quickly, and had a range of kinda herky-jerky iso moves he'd use to clear space. Whilst thin, he had good length, and he used it both to get his shot off, and to cause problems as a defender.
No-one was really shooting threes much back then, but I see no reason he wouldn't have been able to adjust and shoot more if he played in a later era. He did shoot a lot off the dribble (from memory) when compared to someone like Miller, who flew off screens more, so that might have had some impact on his three point shooting I guess.

Whilst different in several ways, there were aspects of his overall game a little similar to Reggie Lewis, in terms of creating the mid-range shot, rangy defence, good and consistent scorer, but not reaching pure killer status on either end. Not sure if that helps you (Lewis was only a little later than Blackman anyway).