It Lives!
From
The Binding of Isaac

Original Game Character: Yes
Humanoid: Creepily enough, yes. It is the physical manifestation of Isaac's mother's "Love"
Female: A.K.A Mom's Heart so yeah
Villain: Indeed, She tries to sacrifice isaac!
Does Not Use Hand to Hand Weapons: Her only attacks include summoning enemies to fight for her, and firing green bile bombs from god knows where.
Main Antagonist: Mom's Heart (a.k.a love for Isaac and her faith toward her religion) are the entire reason for the conflict in the first place! The entire reason why Isaac has to go through Hell and Back (bodaciously) Is due to her.
Blue: Tricky Tricky, thought you almost had me here, but as any good biologist knows, veins are indeed blue. They may look a tad purple as pictured above, but if that is not good enough for you, you are technically fighting It Lives/Mom's Heart in the Womb, meaning you are technically within Mom. Why am I even mentioning this? The first mom boss fight has a blue form that is stronger and more powerful. SO if you want to get TECHNICAL It Lives! is housed within a blue vessel.

However you may cry, "HEY, MOM IS HUMAN ANNNNNND SHE USES HAND TO HAND COMBAT. WHAT GIVES YOU JERK"
And while that may be true, It Lives, and Mom aren't the same entity! It Lives is a delusion induced by isaac, a combination of a monster and his mother's false love that he has to combat.
And actually one could argue that Mom isn't even human either. It is heavily implied that the mom in the boss battle was either A, possessed or B not even real, simply a figment of Isaac's imagination as evidenced by the spoilerific ending I will not share.
Blam. Slam. Kablam
Hot D-D-daisies.
That was tough.