Both the printer’s manuscript and the 1830 edition read blessing, in the singular, which means that the original manuscript very likely also read in the singular. The 1837 edition reads in the plural. This change in number may have been unintentional, especially since it was not marked by Joseph Smith in 𝓟. As explained under 3 Nephi 20:15, in general the text permits either the singular blessing or the plural blessings. In this particular case, one could argue that the singular is correct because it is referring to the blessing of possessing the promised land. In any event, the critical text will here follow the earliest reading, the singular blessing.
Summary: Restore the singular blessing in Mormon 5:19 since 𝓟 and the 1830 edition read in the singular (both are firsthand copies of 𝓞 for this part of the text).