The green color of freshly harvested broccoli is due to the green pigment, chlorophyll. When you heat chlorophyll it may be changed into a yellow or 'pea green' pigment. This also happens as the broccoli ages - there is generally loss of chlorophyll and development of less desirable colors. You can possibly avoid this by reducing the blanching conditions (lower temperature and/or less time) so that there is less exposure to heat. In addition, using younger broccoli will help because there will be more chlorophyll than in older broccoli.
– Diane Barrett