Ao no Miburo: Serizawa Ansatsu-hen

The Black Ships arrive in Kyoto as the shogunate stumbles toward ruin. Living amidst the chaos is Nio, a kindhearted boy on the verge of discovering his destiny. He meets two "Miburo," Hijikata Toshizo and Okita Soji, members of the band of Mibu ronin who will later become the famous Shinsengumi. With dreams of a better world, Nio joins their fight to defend the city.

(Source: Crunchyroll)