I kinda agree. Garnet really isn’t a reliable narrator since she’s telling the event to a young boy and she is made of two heavily biased gems both of whom probably didn’t see blue diamond in private. She might’ve left some stuff out because the way blue diamond was presented seemed odd. For all we know Ruby and sapphire spent years on the earth before they were found by rose and pearl since to gems a year is really no different than a week. I think the episode took place after pink diamond got shattered which would explain why she was so mad at ruby: she basically cost her the only chance they’d have to avenge pink, because the issue was so great it required a team of diplomats to solve.
As for the whole fusion thing, I do kinda agree with several reviewers that fusing for too long like garnet would be a type of unhealthy relationship in real life so that could be a possible reason, since fusing for too long like Sugilite could lead to you losing yourself.
For blue I see her as neutral. Sure she is kinda bad since she robbed planets of life but is it really different than a human country bringing a variety of animals to extinction by mistake? It might be unintentional and as we see with pink diamond they were going to keep a few of them alive. She’s meant to foil pearl after all.