Here is how:
1. Go to your Blackberry Messenger. Press menu, choose Options
2. Under Chats, you need to change Conversation Style to Standard. Bubbles and Stripes have the alternate colors between you and the other recipients with blue and White. So change it to Standard means no alternate colors.
3. Hit and press Save
You should notice improvement over your chat. Give it a shot.
I found this tips from here