I haven't read this at all so no clue if it's the right thing but is it this? Its isekai and the main two characters seem to fit the description you shared.
https://www.mangago.me/read-manga/i_thought_it_was_a_common_isekai_story/
god i went on a hunt to find this again but it should b this one !! ლ(´ڡ`ლ)
https://www.mangago.me/read-manga/the_stereotypical_life_of_a_reincarnated_lady/
omg yes its this one, ilysm ty <3
its not this, but i love this one dearly ty :)
ofcofc its allg, have fun reading ml <3
please oh my god, what the one with the FL who isekaid, and then theres word of a woman from another world coming with black hair and such, and shes friends with the ML but please oh my god and thw FL is terrified of them and thinks shes going to die or something oh my god please, and the crown prince has purple hair and the FL has red hair and the ML has blond hair and the title has something like "average love story" or something like "common contract marriage" OR SOMETHING PLEASE ITS ON THE TIP OF MY TONGUE I CAN LITERALLY SEE THE COVER IN MY MIND and the ML is like, new to love so he shows skin and the FL calls him stupid and dresses him properly or something PLEASE PLEASE PLEASE and the BLACK HAIRED WOMAN IS ACTUALLY FROM ANCIENT TIMES AND IS A SWORDMASTER PLEASE IM BEGGING