The blue Galaxy is a gem, and it behaves and sparkles like one. It works great with a 5 diamond collection. Just make sure you have 2 collections each day if you use it: 1 for the BG and what buildings you want to try to double, and 1 for "everything else".
It's a probability based GB. As you lift it higher, (level it up), you get more chances and a higher probability (charges as Inno calls them) each 24 hours to try for a double on a building. You should start to study your buildings for what they deliver that you want to try to double. Buildings with Diamonds, FP, goods are top candidates to consider.
The BG won't double OTHER GBs, warrior generation, boosts for war, etc. It will also double coins, medals, supplies and BPs (I thinnk).
@ level 68, the BG gives you an opportunity to try for a double on each of 14 of your buildings. That means you have to pick 14 of your best buildings to try to get a the BG to double each one of them.
@ level 91 you get 15 tries a day and a higher probability that your try to get a double is successful. SO you should have 15 city buildings in mind (or 1 more than any level below 91...)
Here's how to read this:
The blue galaxy @ level 67 gives you 14 tries each 24 hours to apply to a building for a double. For each of those 14 tries, there is a 64% chance you'll get that double.
For level 68, there are still 14 tries each 24 hours, but... the probability (chance) you'll get a double goes from 64 to 65%, just a
wee bit better.
Probably best to read a bit on it, in the Inno wiki or what other resources you can find on it from talking with other gamers on their experiences. My BG really shines, and I think most would agree it's a very worthy GB to have.