purple blood parrots are probably dyed (there is a version of blood parrot that is really blood red, and is called the "purple" parrot) if they are flourescent colored. do not buy them. they are going to be stunted and disease ridden, and will die an early death.
real blood parrots are "naturally" an orange color and they grow to about 8 inches. if you eant a blood parrot, gen an undyed one, and you will need at least a 55 gallon.
IME, they dont get very aggressive unless they "breed." i have one with angels, tetras and corydoras and i havent had any problems so far and he is at 6"