Yes. as long as the move is calculated using normal type (no stellar does not count as normal because it has different resistances) the move is boosted by silk scarf. so tera blast can inly get boosted without tera, or tera normal
No, it does not. I tested it in Pokemon Showdown, and there is no boost, there are only boosts from the Tera itself. You can test it for yourself if you want to, it's very simple.
Hope I helped!