The Jellien Glasses are black specifically to mimic the eyes of the Jellien Clones.
Role in episode
As SpongeBob and Sandy wait inside of Sandy's treedome, planning to go to the Krusty Krab, Sandy creates the Jellien Glasses for herself and SpongeBob to evade the Jellien Clones outside. The glasses manages to fool the Jellien Clones and SpongeBob and Sandy make it to the restaurant, where Jellien Mr. Krabs greets them. The Jellien Glasses manage to fool Jellien Mr. Krabs as well until SpongeBob accidentally knocks off Sandy's pair, leading Jellien Mr. Krabs to expose them as imposters.