BL (and BLB) bulbs are a very diffuse light source. It is like enlargers for silver gelatin printing where a diffusion enlarger will give a less sharp and less contrasty print than a condenser enlarger (everything else being equal).
A carbon emulsion is very thick (relative to silver gelatin) and if you imagine a small dot sitting on the top of a carbon tissue with light from a diffuse light source, the shadow cast by that dot ends up spread over a significant distance by light hitting it at a very shallow angle. See the diagram below (okay...it is a pretty rough drawing!LOL!)
But a point source (or close to it) the dot's shadow is straight down into the carbon emulsion with very little spreading.
I made a palladium print of Friday and accidently printed the neg upside down under a bank of BL tubes. Just the thickness of the film base was enough to make the image mush. Same idea.