Like you already experienced (and fixed) with ement.el, the same thing could happen in prism.el.

There are a few instances of (face-attribute 'default :background) which may be "unspecified-bg", which is a color that cannot be blended.

This results in error messages at startup when running emacs -nw in some terminals:

prism-blend: Wrong type argument: number-or-marker-p, nil