Option #1
Uses pure CSS and needs a pseudo-element. This requires inheriting the backround layer responsible for the border gradient onto the pseudo element that then gets blurred.
Option #2
Needs no pseudo-elements, but requires an SVG filter. The basic idea is the same as here - everything within the padding limit is fully opaque, while the border has an alpha in the [0,.5] interval, so the filter can extract the border and the padding-box separately and apply effeccts on just one of them.
Option #3
Similar to the previous one, only also adding a grain effect to the border glow. This involves using a displacement map generated via a feTurbulence primitive.
Option #4
Similar to the previous ones, only also rounding the card box corners and the rotating card border ends.