The Library of Congress has mounted a major on-line Leonard Bernstein exhibition in honor of his centenary, which offers scans of most of the Library’s extensive Bernstein holdings. The displays do honor to Marc Blitzstein’s close friendship with Bernstein by showing over fifty letters from Blitzstein and members of his family.
The entire Bernstein collection is available to view here: https://www.loc.gov/collections/leonard-bernstein/about-this-collection/
Of those materials, many relate specifically to Blitzstein, either correspondence between the two friends, or as mentions in Bernstein’s letters with other correspondents: https://www.loc.gov/collections/leonard-bernstein/?fa=subject:blitzstein,+marc