Here in the printer’s manuscript, Oliver Cowdery initially skipped the conjoined clause “and blessed art thou”. The reoccurrence of the word blessed seems to have led Oliver to skip this second clause. Somewhat later, probably when he proofed 𝓟 against 𝓞, Oliver supplied the clause (the level of ink flow for the supralinear insertion is somewhat heavier). Obviously, there would have been absolutely no motivation to emend the text here by adding the extra clause, which (along with the following “for as sure as thou livest”) seems to be formulaic rather than substantive in meaning.
Summary: Maintain in Alma 19:12 the formulaic phrase “and blessed art thou” that Oliver Cowdery initially omitted when he copied from 𝓞 into 𝓟.