According to UNO ecologist Dr. James Wilson, all of the melanistic/black squirrels in Nebraska are Fox squirrels (S. niger) [personal email correspondence 5.4.20]. Wilson is interviewed in this article about ongoing research into these melanistic variants: https://www.unomaha.edu/news/2020/01/uno-magazine-nuts-about-black-squirrels.php, and published the 2013 paper below discussing their westward expansion out of their historic range along the Missouri River/eastern Nebraska.

WILSON, JAMES A. “Westward Expansion of Melanistic Fox Squirrels (Sciurus Niger) in Omaha, Nebraska.” American Midland Naturalist, vol. 170, no. 2, 2013, pp. 393–401. JSTOR, www.jstor.org/stable/23525591. Accessed 12 May 2020.

In this 2013 paper, Wilson updates and expands upon a previous study/survey of melanistic squirrels by Lueninghoener (Lueninghoener, Edward W., "An investigation of the melanistic phase of the Western Fox squirrel (Sciurus niger rufiventer) in Eastern Nebraska and Western Iowa" (1973). Student Work. 3090.
https://digitalcommons.unomaha.edu/studentwork/3090), concluding that the proportion of melanistic squirrels in the Omaha and Council Bluffs, IA metro areas had expanded in the 40 years since Lueninghoener's survey/study. The existence/expansion of large canopy trees appears to be a key factor in determining melanistic squirrel distribution. Wilson also discusses the hypothesis of a thermal benefit of black/dark melanism in this species, a hypothesis discussed by Lueninghoener as well.

Another interesting issue is the role of human importation/relocation, what Wilson refers to as "citizen relocations" (= not part of any organized wildlife management policy). I've been able to find few details on this; perhaps others will share some in the comments. Wilson refers to Lueninghoener, who explains:

"There is a good chance that imported black phase individuals have altered the population distribution in [Nebraska and Iowa]. The conservation officer in Council Bluffs [surveyed for Lueninghoener's study] knew of many instances where black squirrels were trapped and relocated. The conservation officer at Onawa, Iowa stated that black squirrels had bene imported from Council Bluffs, but so far the stocking had proven unsuccessful.

The above probably explains the populations in three central Nebraska towns, Fremont, Fairbury, and Hastings. Though little is known about Fairbury and Hastings, the Fremont population is supposed to have descended from black phase squirrels imported from Council Bluffs."

(Lueninghoener 1973, page 19).

Former Fontenelle Forest Chief Naturalist D. Andrew "Andy" Saunders included this in his 1985 book on mammals of Fontenelle Forest:

"At least one color variant is common in the Omaha-Council Bluffs area, a melanistic phase, which is more abundant in urban neighborhoods than the rural countryside. During the past decade, black fox squirrels have spread north and west into Omaha, and now occur to some extent in all areas where previously they were absent. They do not occur yet in Neale Woods. They may represent the only population of fox squirrels in North America in which melanism is common; and it is not a recent phenomenon because Audubon encountered them during his travels up the Missouri River Valley in the mid-1800s. Individuals tend to be either all black or completely orange; rarely is one reported that is a mixture of the two. The same is true for litters produced by parents of the two color phases."

Mammals of Fontenelle Forest and Neale Woods, D. Andrew Saunders (Fontenelle Forest Association printing 1985), page 28.

Audubon's "Black Squirrel" (Sciurus niger) from his "Viviparous Quadrupeds of North America" (1845) (https://johnjames.audubon.org/what-viviparous-quadrupeds-north-america).

Lueninghoener (Lueninghoener, Edward W., "An investigation of the melanistic phase of the Western Fox squirrel (Sciurus niger rufiventer) in Eastern Nebraska and Western Iowa" (1973). Student Work. 3090 https://digitalcommons.unomaha.edu/studentwork/3090) notes that Audubon provided an accurate description of melanistic/'black' fox squirrels in his Missouri River Journals (1843), having first observed them near St. Joseph Missouri, and continuing to observe them up until "Council Bluffs" (just north of Missouri Valley, Iowa...north of present-day Council Bluffs across the Missouri River from Omaha, NE). The "black squirrel" was initially identified/named in Audubon's honor (S. auduboni) before later being recognized as only a melanistic form of S. niger rather than a separate species (Lueninghoener 1973, page 2).

Lueninghoener also uses Audubon's observations to argue against the hypothesis that these melanistic/'black' squirrels were likely only found in urban areas: "The fact that Audubon saw the black phase here at that time meant that the black phase was able to survive its natural predators without any help from a city's protection" (Lueninghoener 1973, page 18).

Hi everyone,
This is a basic project to collect all melanistic/'black' squirrel observations in Nebraska. This is my first iNat Project, so let me know if there are features/settings I may have missed/should be including to improve it.
Thanks, Colin