I nominate
Nanno from Girl from Nowhere for being the worst user of blood manipulation.
Nanno is a demon / demi-goddess in the form of a human, whose aim is to be the bringer of karma, punishing humans using very particular methods in accordance with karmic retribution and poetic justice. This is easy for her because she is super clever, has immortality and has many other supernatural abilities. In season 2, she saw potential in another clever girl named Yuri, who kind of outsmarted her for a moment. Yearning a challenge, Nanno decided to grant Yuri the same immortality and paranormal effects she has, letting Nanno have a worthy rival on her missions. This was done by Nanno non-physically moving blood in water, into Yuri's mouth after she had drowned, to make her immortal and bestow her power.
There's no evidence Nanno can use blood manipulation in any way other than moving it in water and being a means of power bestowal. As strongly implied during the
Wonderwall story from season 1, Nanno can bestow her powers into something just by thinking it, so blood manipulation is basically just an extra unnecessary step.
Nanno actively jeopardized her role as the bringer of karma and had put humanity in danger by giving Yuri her powers. Both characters can be cruel, but Nanno prioritizes slow and meticulous methods of punishment that suit people's misdeeds and situations, whereas Yuri's sense of justice is selfish and she doesn't care about who dies if it means victims quickly get the pleasure to triumph over those who wrong them. Nanno is more responsible with the power to avoid consequences and I bet most people would rather Nanno be the judge, but she had put herself in a situation where she had to win a sort of battle of wits in order to maintain being the bringer of karma, risking Yuri revolutionizing the world by spreading Nanno's powers to more bullied psychopaths.
I know it's technically useful to be able to use blood manipulation for power bestowal that grants a long list of abilities, but until someone finds a worse blood manipulation user, I think Nanno is a good choice to list as the worst one. In summary, the only advantage she can get from her blood manipulation is power bestowal that she doesn't need blood manipulation to perform anyway, and the canonical way she utilized blood manipulation was to her disadvantage.