Kongs are wonderful and I use both the red and the white in two sizes but like all toys knowing they can cause problems is a must.
Kong blue
RADIOPAQUE material - visible on x-rays ( yes some dogs can chew the orignals up)
Higher tensile strength, 25% stronger than regular Kongs
More puncture resistant than regular Kongs
Stretches more than other rubber toys on the market
Natural Rubber
(NEW) Kong Blue Why Kong Toys Are The Best Toys For Dogs - The Fun Times Guide to Dogs
Read bottom of page above as to what Careasa said about the new blue kongs