Why is the presence of blue stragglers surprising?
Blue stragglers seems out of place in open and globular clusters.
Stars in open and globular clusters normally follow a particular curve on the H-R diagram. Blue stragglers are unusual that they are bigger and bluer than the rest of the stars (See picture for reference).
Blue stragglers tend to occur in denser regions of the clusters. They also have masses two to three times that of the other stars. This suggests that they were formed from two or three stars which collided and merged.