I've kept the water pretty clean, and I feed him a twice daily diet of blood worms. I'm thinking it could be because of stress? I heard a betta's colour is a reflection of its health or happiness / emotional state.

Any fish that changes into brighter colors is usually due to a good diet and exceptional environment. If he starts turning a dull color then I would be worried. It can also be due to sexual maturity. Usually as fish get older thier bright colors fade. An aqua blue is a bright color so I wouldnt be worried.

Changing from red to blue is probably a natural genetic thing as he is maturing. pets question and answers,www.5d2d.com

Fish can go dark, or fade to lighter colours under stress, but to actually change from red to blue, it will be something genetic, like peoples hair changing colour. I used to be blonde, then brown, now grey, although the grey may be related to stress of having blonde children around....

Ian

He could still be a young Betta and is just growing up, thus changing colours, or, as the above answer said he could be getting older, i remember one of my white Bettas as he got older started to become grey =P

fish often change color as they get older. research betta care and see if you're meeting all his needs. if yes, don't worry about it.