Looking at our current roster, Blatche and Garnett are both playing power forward but they are both better suited to play center. While Blatche is talented enough to make up for it playing against opposing benches, especially offensively, he's too inconsistent to have as a starter. While Teletovic has a different game than Blatche, he also simply isn't gifted enough defensively to start.
There are three things I believe we need out of a PF: pick and roll defense, a consistent mid-range jump shot to clear the paint for Lopez, and rebounding. Darrell, at age 25, has two out of the three. He's a great pick and roll defender and has been a killer mid-range jump shooter this season. He could run a pick and pop or a pick and roll with Deron. He's been so consistent that Shaw called him his teams "safety net."
The best thing about Arthur is that we could conceivably have a shot at him this summer without giving up any assets. He's on a team with a crowded frontcourt making mini-MLE money this year and has a player option for next year. It wouldn't be too far-fetched to think he'd give up ~200k for a starting spot on a playoff team in a major media market. There is a complication in that he is his agent's only NBA player so might not be so ready to take less money.
Alternatively, we could trade Teletovic for him this season, but I'd keep Tele since Arthur's range doesn't extend all the way out to the three point line and there's no guarantee that Blatche will stick around next year.
Here are a couple articles about Darrell Arthur from Roundball Mining Company: