The addition of "blind" goes back at least to 1823:
https://archive.org/details/publicparlourcot00harr/page/38/mode/2upI suspect there are earlier instances, possibly in earlier camp meeting songsters. Anyway, it clearly, as you point out, revolves around how Bartimeus was sung, but not necessarily its "correct" pronunciation. I'm guessing that, in America at least, it became common to pronounce (or at least sing) it as Bar-ti-meus, with "meus" contracted and pronounced as a single syllable, thus "requiring" the addition of "poor: to fill in the 7 syllable line. The tune CHARLESTOWN (originally named CHARLESTON) was first published in Pillsbury's
The United States Sacred Harmony in 1799, where it was paired with "Come thou fount of ev'ry blessing." I believe the earliest appearance of this tune with the Newton text was in Carden et al
The Western Harmony in 1824. I don't have access to this collection, but I suspect it used "Thus poor blind" and treated "meus" as a single syllable. This is certainly the case with CHARLESTOWN when it was added with this text to
The Southern Harmony. Something in the back of my mind hints that CHARLESTOWN might have been added in the 1837 printing of
The Southern Harmony, but it's definitely there in the 1840 edition, which is available at IMSLP (it's on p. 23b):
https://ks15.imslp.org/files/imglnks/usimg/a/a7/IMSLP559075-PMLP213802-Southern_Harmony_nobts.edu_1840.pdfI feel that the long history of the "poor blind" version with "meus" sung as a single syllable in connection with CHARLESTOWN/CHARLESTON justifies not making a change. If such a change were made, the other two tunes that use this version of the text (VILLULIA 56b and FRIENDSHIP 458) and hundreds, perhaps even thousands, of singers would be perplexed. Perhaps Will Fitzgerald or another member of the text team can comment on whether this issue was discussed during the editing process.