That would be Claymore
it's set in a fictional medieval island where humans are plagued by Youma, humanoid shape-shifters that feed on humans. A group known as The Organization, creates human-Youma hybrids to kill Youma for a fee. These female warriors wear armored uniforms with white body suits. The public refer to them as "Claymores" alluding to their Claymore swords, or "Silver-eyed Witches," due to their silver eyes and how they have powers beyond humans.
Evidence
it already has more than 50 chapter
According to Wikipedia the manga has 143 chapters and is still ongoing
The genre is action, and have some explicit content with lot of blood
I've only seen the anime but i believe it's action and quite bloody however the explicit content in the anime also has a bit on nudity in it, there's a scene of cannibalism and one of the characters almost allows herself to be raped
The main character is a woman, short hair, white hair, and have sword
That would be Clare (Pictured Above)
This woman is a monster slayer, wandering between cities to slay a monster, and the citizen pays her. But she's a monster herself.
Claymores go from City to City, town to town killing Yoma , they leave but before doing so notifies the people that a representative of The Organization will appear to collect the fee. all Claymores are Part Yoma , Clare herself is part Claymore because she was infused with a Claymore's Head (the woman who raise her)
At the beginning, a boy decide to follow her, for what reason I don't really remember, perhaps to revenge his parents death.
this boy is Raki who she saved, he at first begins to follow her but then Clare decides to take him along seeing herself in him when she was saved by Teresa, the Claymore who saved her from Yoma
One chapter reveal that she has a friend who also the same monster slayer like her, but she has to kill her, because her friend has been consume by the monster inside her, and turn into a monster.
As said above Claymores are half Yoma, their powers and strength come from using this side of them by using demonic energy know as their Yoki Aura, if they use too much they "Awaken" and becomes extremely power Youma known as Awakened Being. it's explained that awakening is likened to the feeling of sexual climax and Women was shown to resists this more than Men so it's the reason why Claymores are women, however later in the series Clare learns of the Male Claymores having existed however it's assumed they are all Awakened Beings now, Claymore tend to hunt each other down in order to prevent them becoming Awakened Beings.
Contradictions
It's seinen
Wikipedia says the demographic is Shōnen
