Every Bloomie gets a rank when it hatches, from F- (weakest) up to S+ (best). The rank never changes, so it is worth hatching lots of eggs to find the good ones.
A higher rank means higher final stats and a higher selling price. The jump is biggest at the very top: an S+ Bloomie ends up with far better stats than an A+ one.
F- to F+Weak
E- to E+Below average
D- to D+Okay
C- to C+Average
B- to B+Good
A- to A+Great
S- to SExcellent
S+Legendary
The colors above match the outline your Bloomies get on the desktop (you can switch this in the options). S+ Bloomies glow in rainbow colors.
Tip
Use your Incubator's Sell if rank lower than setting to automatically sell everything below the rank you care about. Your box stays tidy and you still get the gold.