Outaishi Denka wa Koukyuu ni Uranaishi wo Goshomou desu

Forte, the daughter of a duke who works as a fortune teller while hiding her true identity, is falsely accused one day and is threatened by the crown prince's aide and ordered to "find the mastermind behind the murder of the prince and queen among the three noblewomen in the harem!" Faced with the prospect of being jailed if she fails, Forte disguises herself as a maid and begins investigating the harem, where she is aided by the mysterious black knight Alto. But what is his true identity, the man who knows so much about the royal palace...?!

(Source: Kadokawa, translated)