I input the word "blackface" at newspapers.com and they claim 16,334 hits for the word in Pennsylvania between 1900 and 1920. newspapers.com is a pay website.
Dear Mr. Reid,
Thank you for posting your request on History Hub!
We suggest that you contact institutions with a focus on state and local history to include Pennsylvania Historical and Museum Commission; the Historical Society of Pennsylvania, county historical societies; the African American Museum in Philadelphia; and universities in the area such as Dickinson, Shippensburg, Central Penn College, Elizabethtown College, Franklin & Marshall College, Messiah College, and Wilson College.
We hope this information is helpful. Best of luck with your research!