I hate the comparison, because the skill sets are very different.
Things Judo BBs are usually good at on the ground:
1. Attacking the turtle
2. Holding a pin
3. Passing half guard to get a pin
4. Not getting submitted, turning to their bellies to avoid pins
5. Attacking chokes and arm locks in transition from standing to the feet
Things blue belts are usually good at (relatively speaking):
1. Basic closed and open guard passing
2. Basic closed and open guard play
3. Basic subs from superior positions
4. Submission defense
5. Escapes
These are two pretty distinct skill sets developed in reaction to two very different competitive rule sets. Direct comparisons aren't that useful. Typically, a good Judo BB will have much, much better overall grappling awareness and skill than a blue belt, but will have gaping holes in their knowledge of what would be needed to win BJJ matches. I would never give a Judo guy a blue belt as soon as he walked in the door, but I would expect him to pick up BJJ significantly faster than I would someone with no grappling experience, just as I would a good wrestler or Sambist. A lot of what makes a person good at grappling are the awareness, balance, reflexes, and instincts that are not style specific. My BJJ coach has no rank in Judo, but he's damned hard to take down because he has great balance, grip fights well, and more or less instinctively doesn't let you get good position on him (also as soon as you shoot he loop chokes you...a different problem).
When I do BJJ style ne waza with nationally ranked Judoka who outweigh me by a 30 lbs, I murder them. Not because I'm a better grappler overall (they throw me around plenty), but because I have a better understanding of that meta-game and better command of the specific skills required to succeed in that domain.