I've found the best way is to use a checkerboard pattern and use either Jacob's ladders or fertilizer in one of the empty spaces. Then water only the flowers you want to breed. Don't place flowers immediately next to each other. Hopefully you have beautiful town ordinance otherwise it will be harder to breed them.
Also, make sure your blue roses are legit. I once got blue roses from someone on tumblr and they did not breed blue roses for the longest time. I used fertilizer, planted them diagonally, used Jacobs... nothing worked. I only kept getting red or white roses from them until I found out they were most likely hacked. D: After a while, I bought legit ones and they started breeding way more. And like Cass123 said, make sure you have the beautiful ordinance and to plant them diagonally.