Optional background: The Pnei Yehoshua Megila 21a asks on Rashi's comment that the Mishna permits sitting for megila lechatechila- 'that it would seem that the bracha should also be sitting (PY doesn't exlain why). This goes against the Yerushalmi that the Beis Yosef brings that all blessings on mitzvos are said standing?'
The PY answers by limiting the Yerushalmi to cases where the mitzva is done standing- those that we learn from Omer (Bakama/Bakoma+lecha)*. But mitzos that are done sitting, the Yerushalmi would say to sit for the bracha.
Magen Avraham (OC 8:2) reads the Shulchan Aruch that both the mitzva and bracha of Talis should be done standing and asks from the mishna by chala which implies that an undressed woman may make a bracha if she's sitting. The MA answers that Teruma is not such a mitzva since it's only to fix the food. The PY assumes that his question is both on the bracha and the mitzva and doesn't understand the MA.
My take on the arguement (take it or leave it): The PY holds that while the mitzva is the primary entity, a bracha was established prior to the mitzva, as if they are a single unit. If you sit for the primary mitzva, you sit for the bracha. The Yerushalmi was commenting specifically on the standing mitzvos. The MA holds that while brachos were established prior to mitzvos, they are their own entity of hoda'a (thank You for sanctifying me with this mitzva). We must stand for hoda'a- just like the Yerushalmi says (also, see the end of PY re hallel). [Alternatively, it's an avoda and we stand for avoda- see Bach.] There are a few exceptions like chala, teruma, shechita- where you're not really sanctifying yourself like Talis, etc., your just fixing something in the food so you can eat it. There is still a bracha, but it isn't really a hoda'a, we just keep the same format. So you don't need to stand.
For those who don't want to waste their time: The Pnei Yehoshua says that you look at the mitzva. If the mitzva is sitting, the bracha is sitting. The Magen Avraham holds that you always stand for birchos mitzva (with a handful of exceptions).
It would seem based on Ashkenazi custom by megila (stand then sit) that Ashkenazim go with the Magen Avraham approach. Sepharadim by tefilin seem to use an approach similar to the Pnei Yehoshua.
I would be curious to know what Sepharadim do by megila.
[*Neither the BY's Yerushalmi nor the drasha of Omer appear in our talmudic texts]