When is the best time to take a photo? Trick question! I don't believe there is a perfect time exactly. This thought is most associated with landscape and nature disciplines rather than studio work, but some of this might be applicable to the the most "perfect" of studio setups.
The photo seen here is one that was taken well after the height of blue hour. Even though I was only a few hundered yards from my tripod, had I left to get it and set it up, I would have missed this...
What was more distressing was that I had to free hand the shot to get the composition I wanted. Thus, I couldn't rest the camera on a railing or my knee. I just happened to decide to take a stroll down to the beach after dinner and a few "adult beverages". On my way out, I grabbed my camera out of habit.
In this case, the "adult beverages" helped steady my hands enough to get this shot, but that is an aside to the grander point I am trying to elaborate.
You cannot always be a Boy Scout photographer, you will never always be prepared. Do not waste time when you see something spectacular before you. Never assume it will become more spectacular or be just as while you stop to fiddle around.
Just take the shot!