The reason for it to be blue is simply because we didn't find the talent and the time to work on our artwork.
As you said the important is the user experience. At the moment we're using the best blue artwork we found from places like art.gnome.org and gnome-look.org.
We would love to develop our own theme, maybe a blue one with hints of icy mint.. or a green one, our icons and splash screens... but nothing is done at the moment.
The important thing as well is for the desktop to look nice. So we much prefer an unbranded blue theme than an unprofessional Linux Mint one
Hopefully in the future we'll be able to use Mint artwork that actually looks nice. We're not great at drawing things though
Thanks for your feedback.