Birshut HaRav: In Simon 581:3, the Mechaber writes: "We do not say tachanun on the day before Rosh HaShanah during Shacharit even though we say tachanun as part of the selichot." The Rama adds that we also don't blow shofar on Erev Rosh HaShanah. Obviously we been blowing shofar the entire month of Elul. The Mishna Brura (24) explains two reasons for this. In order to make a distinction between the, the shofar blasts of Rosh HaShanah that are a Torah obligation versus the shofar blasts that are optional or that are by Rabbinic tradition that we add during the month of Elul. In other words, we're differentiating between the two levels of mitzvot in the blowing of the shofar. Even if the first day of Rosh HaShanah is on shabbos you still don't blow shofar on the day before Rosh HaShanah. You might have thought that you would blow shofar on Friday because you'll have shabbos when you won't blow shofar which will differentiate between the Rabbinic shofar blasts of Elul and the shofar blasts that will be on Sunday, the second day of Rosh HaShanah. That's not the case, we still don't blow shofar tomorrow morning. Nonetheless it's permissible to blow shofar for the purpose of learning because it's sufficient that we don't blow shofar in the synagogue itself.
That's the first reason we don't blow shofar on Erev Rosh HaShanah, basically to differentiate the two shofar blasts, those of Elul and those of Rosh HaShanah.
The second reason: to confuse the Soton. If that is the case then we shouldn't blow Shofar at all on Erev Rosh HaShanah, whether it's just for the purpose of practicing or whether it's in the shul. The Elya Raba permits practicing in a closed room, meaning a completely private place. Nonetheless, presumably the Soton will still be confused.