Last season I grew cornflowers in a pot,wild seed never fails to germinate.I bought the seeds from Thompson and Morgan,they were red white and blue. Another idea for the blue colour is an easy to grow hardy annual,Phacelia tropical surf its common name is californian bluebell,also available at the same place,just sow where they are to flower April to May,bear in mind they grow to about 12 inches,they are hardy annuals.