Kuro Gal ni Natta kara Shinyuu to shitemita.

College students Shion Chihara and Rui Chihaya are childhood friends who enjoy picking up girls together. When a strange drug transforms Shion into a woman, Rui cannot resist sharing an intimate moment with him, even after learning his true identity. As they investigate Shion's new body and the events that shift him between male and female forms periodically, their relationship deepens in ways that they never before thought possible.

Plot summary from MyAnimeList. Data and images via Jikan API.