Radial Blur seemed to be the most effective. I also threw 'blur more' and 'gaussian blur' at the workshop but they didn't make a huge difference. I haven't touched the area near the border. If I got too close, it smudges the black border as well.
A shot of the monastery at Sucevita for context. The paintings are too far away for detail to be discerned and the detailed painting shots don't convey the context of the Monastery buildings. What a conundrum!
I tried a quick workshop adding contrast & saturation to the top, reducing brightness and giving the sky a cyan tint as well. I didn't have time to guillotine the borders and replace them again after. You can see the effect on the white borders.