Lovely Blue is one of the Horses of the Rainbow. She was originally released in May 2015.
She can breed once with Shiny Yellow to give birth to a unique foal: Forest Green.
She gives her owner 10 passes every Friday (after the daily update), provided you logged in at least once over the previous 3 days. For VIP accounts, horses will receive double morale from being groomed on Fridays.
Once you own all the 7 Horses of the Rainbow, you can obtain Secret Rainbow. This mare cannot be sold. Her skills can be increased by using passes.