Why so much blue for the dark themes?

Well for the solarized dark theme that is simply the color pallete.

I don't know why it was decided for the black theme, but I can have a guess. Blue is one of the few colors that contrasts nicely with black keeping things readable and in general easy on the eye.

Anyway, here is css that you can put in stylus to replace the blue with red colors.
(Make sure to use stylus, don't use stylish as it is no longer open source and has gotten a bunch of tracking added)

.theme-black .nav .nav-item a,
.theme-black .btn,
.theme-black a {
    color: #d22626;
}

.theme-black a.link-user:visited,
.theme-black a.link-group:visited {
    color: #b1222e;
}

.theme-black a:visited {
    color: #c46c6c;
}

.theme-black .btn,
.theme-black .topic-icon {
    border-color: #d22626;
}

.theme-black .btn.btn-primary {
    background: #9a1c1c;
    border-color: #9a1c1c;
    color: #ccc;
}

.theme-black .tab .tab-item.active a, 
.theme-black .tab .tab-item a.active {
    border-bottom-color: #d22626;
    color: #d22626;
}

I just want a straight up AMOLED Black Theme so it looks better and uses less battery when on mobile.

I personally love the look of the black and blue but it's definitely a pain to read.