In Victorian times the yellow rose meant jealousy. This is not a common meaning today. A gift of yellow roses signifies friendship as well as joy and delight. A sunny yellow bouquet can also be interpreted as warmth and welcome. Yellow does not typically imply romantic love; this color leans firmly on the side of friendship.
Blue roses don't occur naturally; they are created. It is not unreasonable to take blue to mean mystery or the need to keep a secret. Blue can also symbolize new beginnings or new opportunities. A blue rose can also convey uniqueness. A blue rose of a lighter shade can denote the beginning of love or enchantment.
Using the most common color interpretations of friendship and opportunity, a yellow bouquet with one blue rose could make up a congratulatory bouquet for a friend who has started a new job. Using the blue to signify uniqueness, you might consider a yellow with blue bouquet for a creative and different friend on her birthday. Since blue could also mean the beginning of love, don't be surprised if the friend that sent the bouquet turns toward romance.
The number of roses can also have meaning. A single rose implies devotion. Eleven roses says that you are loved; a bouquet of 11 yellow roses and one blue one can signify devoted love. If you like the idea of a blue rose for your intended recipient, you can create the blue shade at home. Cut the stem of a white rose and put it in water that contains blue food dye. The natural capillary action of the flower will pull the blue up to color the petals.