Sheeeesh, that's almost a bit kinky...

...
Jeff Jones, hmmm...well, I thought, I didn't know him/her, but I mainly just forgot. Slowly some memories are returning. Seems to me like Frazetta was heavily inspired by Jones. It's almost like a coin toss to say, who did a better job?!

...but I feel like Jones may have been a wee bit more authentic. Still appreciate both and find it odd that I forgot about him/her.

Anyway, the key to either of their styles, including Boris, is to leave the background as un-detailed as possible, like suggestions only, focus almost entirely on the figure (protagonist) and whatever directly interacts with them. Make sure the background also has far less contrast...that's where the whole "fog" sensation comes from, but that may not have to be fog or haze and could be a mixture out of depth of field (out of focus), different lighting (vanish in the dark or washed out by too much light)...many possible approaches to let the background vanish. Put stark contrast only on the figure and it's direct elements (weapon, chalice, anything it may touch or wishes to touch). And try to keep it somewhat monochromatic, like reduce the color spectrum to around where you wish to have the figure, which also should have color-wise the most strength. That often comes from the lighting idea, too. The most light shows the most color.
Also, you may notice that on these old covers the camera hangs fairly low, having to look up at characters and won't show much floor. This way there's less near the main character that might distract from it.
So, yeah... they were very, very unashamed back then, leading the focus exactly to where they wished for it to go and without compromise. Even in group scenes, they would simply plunge the group into darkness and only have full light on the main subject.