Good twist on this passage!
The scale is definitely debatable, but here's what I have:
late 19th/early 20th century African American Historians perspective can be seen as:
1: transnational perspective
2: their own version of a national perspective
Here's a quick passage map:
P1: AA historians (of this period) adopted transnational perspective
P2: Reasons why (1. AA citizenship uncertain; 2. emigrationism hot topic)
P3: Other reason why
P4: AA historians actually engaged in their own version of nationalism & explanation