After a distinguished professional career, he retired in 1977, and in 1978 he discovered the works of J. R. R. Tolkien, which were to be the focus of his energies for the remainder of his life.

[1] Over the next twenty years, Blackwelder amassed a large collection of Tolkien-related books and other materials, which he sorted and indexed.

The Blackwelder Collection, donated to Marquette University in 1982, is believed to be the largest single body of secondary sources on Tolkien ever to be developed.

He married Ruth MacCoy in 1935 and from June 1935 – March 1937 collected 50,000 Coleoptera and other insects in the West Indies as a Walter Rathbone Bacon Travelling Scholar for the Smithsonian Institution.

Although many European and American entomologists had worked extensively on Neotropical beetles no previous attempt had been made to list the entire fauna.

To this Blackwelder added species listed in publications identified in the Zoological Record and where this proved incomplete he undertook the literature search himself.

The Blackwelder checklist is the basis of many subsequent but partial lists and for some groups or regions is the only source of such information.

The knack for organizing and categorizing that drove Blackwelder to excel in taxonomy also colored his post-retirement passion for Tolkien's legendarium.

The Blackwelder Collection consists of "ten linear feet of documents in 140 ... three ring binders, ... over 1,200 volumes ... and over 70 theses and dissertations.

[7] In 1987, Blackwelder established the Tolkien Archives Fund at Marquette University to catalog Marquette's manuscript collection, sponsor public programming, and to provide support for the acquisition and preservation of Tolkien research material in the Department of Special Collections.