The clusters are blue because they are very young--and very hot. The most recently formed are thought to be only a few million years old, a mere blink on the astronomical time scale. Astronomers believe they can use these new stars to develop an accurate time scale for determining the age of stars growing older and dimmer.
The Hubble has also shown that the stars in these crowded clusters, where as many as one million stars can be packed into a swarm only about 20 light-years in diameter, do not necessarily age gracefully. Indeed, they frequently bump into each other. One result of these collisions, say Hubble astronomers, is the small number youthful looking stars in aging globular clusters. These "blue stragglers" are apparently formed when two older stars get a new lease on life by merging.