Is light blue rare? The simple answer is no.
When buying pearls in lots, there are a few different considerations including; size, quality (spots and luster), shape and color. Giving equal consideration to size, quality and shape, the value of the lot will increase as the color becomes darker.
In other words, the cheapest lot will be the light-color lot. Then the lots can be separated by medium-light, medium-dark and dark. All considerations equal, including overtones, the cost of the lots will increase as they become darker.
So it is difficult to say that a light-blue lot is always more or less valuable than a green lot. If the green lot is darker, it is going to be more valuable. But most green lots are darker, and most silver and silver-blue Tahitians come from light-color lots - the least valuable.
I have seen, however, some fancy silver-blue lots. The strand of pearls Cathy chose in this post (second photo
https://www.pearl-guide.com/threads/guess-who-came-to-visit.3900/page-2#post-91969) from the 'other subjects' board, was silver-blue, but has strong peacock overtones. In my opinion, it is just as beautiful as a dark-peacock strand, and even more unique, but the lot we created this strand from was less expensive than a similar dark-peacock drop-lot because the color was medium to light.
Edit: I added a picture of Cathy's strand below because you have to be logged in to see the post from the 'Other' board.