On a technical point, "Blest" was a standard form used in poetry and song; it isn't intended to represent dialect, but to indicate that the word is to be given one syllable rather than the two which might be inferred from "Blessed".  Of course, to confuse the issue, "Blessèd" was often used to indicate a two-syllable value; it's one of those inconsistencies which plague poetic orthography.  The whole thing is of no real consequence in the grand scheme of things.  And please, Toadfrog, do call me Malcolm; however old I get, "Mr. Douglas" will always be my father!

Malcolm