This is an example of inversion after a preposition. John called his mother every day. John called his mother every day as did his sister Susan. John called his mother as frequently as did his sister Susan. Lots of information about fronting and inversion here. See section 2.3 for as+inversion. http://random-idea-english.blogspot.com/2014/09/exploring-inversion-and-fronting.html

It is not an inversion or negation at the end of the sentence "as did" = the result was Equal/the same. Other ways to say same sentence: "The blind came up with the same meaning the same number of times as the sighted" "As a group the blind were able to reach the same meaning as the sighted" "As a group the blind were equal to the sighted" "As a group the sighted were no better than the blind" "The sighted were no better at reaching the same meaning [as the blind]" "The blind came up with the same meaning as frequently as the sighted" Your sentence is using a structure where two structures "at least" plus "as did" are not really needed. You can also say: but as a group (the blind) they generally came up with the same meaning at least as frequently as sighted subjects. but as a group (the blind) they generally came up with the same meaning as did sighted subjects. At least is using this definition: if nothing else (used to add a positive comment about a generally negative situation). "the options aren't complete, but at least they're a start" AS DID = another assertion confirmation of the results "as did I" is another common formal and slightly older way of saying "so did I" I also" In your sentence it means they the group also.