The Shulhan Aruch (O.H. 8:14) states that if one removed one’s Talit, even with the intention to put it back on shortly afterwards, one must recite a new blessing when putting it back on. The Rama (ibid.), on the other hand, says that when one has in mind that one will put the Talit back on, removing it does not constitute an interruption, and therefore one does not recite a new blessing. The Shulhan Aruch (ibid:14) goes on to say that if one’s Talit slipped off unintentionally, one would only recite a blessing if it came off completely, but not if it came off partially.
The Kaf HaHaim (O.H. 8:58) cites the Ritva and Rabbenu Yona, who say that there is a solitary opinion not to recite a blessing even if the Talit slipped off completely, and invoke the concept of Safek Berachot Lehakel (avoiding the recitation of blessings in questionable situations). Rabbi Ovadia Yosef (Yabia Omer, vol. III, Yore De’a, §17) and Rabbi Ben Zion Abba Shaul (Or LeZion, vol. II, ch. 42, §11) concur and say that since there is a doubt in the matter one should not recite a blessing.
Although not explicitly stated in the writings of the Moroccan Poskim, they generally did not go out of their way to side with a solitary opinion which invokes Safek Berachot Lehakel. The Siddur Bet Oved even cites on opinion of the Magen Avraham which says that if one’s Talit slipped off and one was able to catch it by the Tzitzit there is evidence that one would need to recite a blessing. Therefore, all the more so if the Talit came off completely, one would need to do so. The Ben Ish Hai (Od Yosef Hai, Parashat Bereshit, § 3) also suggests that one should recite a new blessing if one’s Talit comes off.
Rabbi Ben Zion Abba Shaul offers a partial solution to this matter and says that if one’s Talit comes off unintentionally near the end of the prayer, one should simply leave it off and not enter a questionable Halachic situation. Another practical suggestion is to make sure that the Talit is properly sitting on one’s body in such a way that it is not likely to come off.
Summary: If one removed one’s Talit with the intention of putting it back on within a reasonable amount of time, one should not recite a new blessing over the Talit. If one’s Talit came off completely unintentionally, one should recite a new blessing.