As of 2023, we can estimate (based off of IBJJF registered black belts) that there are approxiimately ~200,00 black belts in bjj worldwide.
The best bjj belts are 1.Fuji 2.Gold BJJ 3. Hayabusa 4. Kingz 5.Vision 6.Vulkan
A blue belt should know the positions, goals, and common submissions of Brazilian jiu jitsu. They would likely have the upper hand vs anyone entirely untrained.