Here the printer’s manuscript originally had the plural blessings. The 1830 edition has the singular. In his editing for the 1837 edition, Joseph Smith corrected 𝓟 to the reading of the 1830 edition. He crossed out the plural s with the same speckled ink that he used elsewhere in this part of 𝓟 to make corrections for the 1837 edition. So the corrected reading in 𝓟 was not made earlier—that is, not by scribe 2 nor by Oliver Cowdery. We must therefore consider which reading—the singular blessing or the plural blessings —is most appropriate in this passage.
There are many examples in the history of the text where the grammatical number for nouns has been mixed up. When we compare the error rates for scribe 2 of 𝓟 and the 1830 typesetter, we find that their tendency to permanently mix up the singular and plural forms for nouns is about the same. Scribe 2 wrote down only about 15 percent of the text of 𝓟; but if he had done the entire text, I would estimate he would have made about 94 errors involving adding or omitting the plural s (this estimate is based on the 14 errors of this type that he made for his 15 percent of 𝓟). This error rate is a little more than the 1830 compositor’s; as described under 3 Nephi 10:13, John Gilbert made about 85 changes involving the plural s for the entire text. So in order to determine which of the two, scribe 2 of 𝓟 or Gilbert, is responsible for any difference in grammatical number between 𝓟 and the 1830 edition, we will mostly have to rely on internal evidence (that is, usage elsewhere in the text) unless, of course, we have specific information about mix-ups for the particular noun in question. But for the noun blessing there are no other passages where scribe 2 or the 1830 compositor (or Oliver Cowdery, for that matter) mixed up the number.
When we turn to internal evidence, we find that there are three instances in the current text that refer to “receiving blessings” (that is, all three read in the plural), yet in the third case the earliest text read in the singular:
In Mormon 5:19, the 1837 edition replaced the original singular blessing with the plural. In that case, 𝓞 most probably read in the singular since both 𝓟 and the 1830 edition read in the singular (from Helaman 13:17 through Mormon, as already noted, both 𝓟 and the 1830 edition are firsthand copies of 𝓞). The change in the 1837 edition could well be a typo rather than a conscious correction since Joseph Smith, in his editing for the 1837 edition, did not mark that change in 𝓟. Of course, the original manuscript’s singular might have been an error.
One could argue that the original use of the singular in Mormon 5:19 was due more to the use of the verb reserve than the verb receive that occurs in the following relative clause. But there are no other examples in the text of the verb reserve taking blessing(s), so it is difficult to make a judgment regarding expectations when the verb is reserve. But generally speaking, for other verbs we get examples of both the singular and the plural for blessing, as in this contrastive pair involving the verb bestow:
In the case of Mormon 5:19, it is worth noting that Lehi, in his farewell to his children, said that he would either leave his (first) blessing upon Laman and Lemuel and their descendants or he would take it away, depending on them:
And in Mormon 5:19 it appears that Mormon is referring to this blessing that Lehi left to the descendants of Laman and Lemuel, namely, that it was being extended to the Gentiles because the Lamanites were not yet worthy of that blessing. Thus the use of the singular is appropriate in Mormon 5:19, and the critical text will maintain the singular blessing since it is the reading of the earliest text (in this case, both 𝓟 and the 1830 edition).
Generally speaking, unless a specific blessing is being referred to, the Book of Mormon prefers the plural blessings, as in the examples in 2 Nephi 1:10 and Alma 9:23 that involve the verb receive (listed earlier in this discussion). When we consider the other passages that refer to the blessing(s) that the Gentiles will receive, we find that in the general case the plural occurs:
On the other hand, when the Lord refers to a specific blessing such as “the pouring out of the Holy Ghost upon the Gentiles”, the text reads in the singular:
Contextually, here in 3 Nephi 20:15 there seems to be only a general reference to God’s blessing of the Gentiles. Thus the odds favor the plural blessings for this passage, and the critical text will therefore accept the plural reading in 𝓟 rather than the singular 1830 reading. Nonetheless, the singular is still possible.
Summary: Accept in 3 Nephi 20:15 the plural reading blessings, the reading of the printer’s manuscript, rather than the singular blessing, the 1830 reading; also restore in Mormon 5:19 the original singular blessing, the reading in both 𝓟 and the 1830 edition; the Book of Mormon text prefers the plural when no specific blessing is mentioned, although the singular is also possible in general contexts.