A blue Moon is the second full Moon in a month. It is extremely rare and therefore magical. Unfortunately blue Moons appear about once every three or so years so you might have a long wait before casting your love spell.
On the night of the blue moon go out into your garden and stand in the moonlight, then chant:
Blue Moon shining in the night,
Fill my heart with love and delight,
Grant (name of the one you love) loves me too,
Bless our love that is true.
And if it harm none, so mote it be.