As described under Mosiah 2:24, there are a few places in the manuscripts where Oliver Cowdery intended to write are but accidentally crossed the e, giving art. In each case, as here in Alma 27:12, the t looks as much like a crossed e as an actual t. In this instance, the incorrect art was copied into 𝓟 and then into the 1830 edition. In his editing for the 1837 edition, Joseph Smith grammatically emended the art to are.
Elsewhere in the text there are 40 examples of “blessed are” followed by a plural subject; not surprisingly, the manuscripts consistently read are rather than art for each of these other examples. (Here in Alma 27:12, are is possible since this people is a semantic plural; see the discussion under Alma 24:30 regarding the singular or plural number for the word people.) There are also 14 examples of “blessed art” in the text; all of these are followed, as expected, by thou.
Summary: Maintain in Alma 27:12 the correctly interpreted “and blessed are this people”, even though what Oliver Cowdery actually wrote in 𝓞 was “and blessed art this people”; in a few instances, Oliver miswrote are as art by accidentally crossing the e as if it were a t.