I finally found a way to change the background color in-game for Blackthorne!
If you edit the savestate file from ZSNES (.zst or .zsx (where x is a number)) with a hex editor, the background color is located at position 23571 (5C13h) and 23572 (5C14h). You'll see "00 00 39 67 00 00 42 08 C0", where the first "00 00" is the background color. Change that to "1F 7C", save the changes, open the game in ZSNES, and load the save file you edited (pos 0 = .zst).
Now just disable the backgrounds (key 1 and 2) and you'll see the sprites with a pink background!
If you edit the savestate file from ZSNES (.zst or .zsx (where x is a number)) with a hex editor, the background color is located at position 23571 (5C13h) and 23572 (5C14h). You'll see "00 00 39 67 00 00 42 08 C0", where the first "00 00" is the background color. Change that to "1F 7C", save the changes, open the game in ZSNES, and load the save file you edited (pos 0 = .zst).
Now just disable the backgrounds (key 1 and 2) and you'll see the sprites with a pink background!