Since he is so tame gently search the area where you are noticing dried blood, move the feathers aside in that area to check for any injury. Many birds who have had too severe of a wing clip in the past will chew and bite at feathered areas when new feathers are growing in. It is believed that it may cause a bit of itching to the area which can make them use their beak to pick at the area. Sometimes this will cause damage to a blood feather which can bleed, most birds will go ahead and pull a feather out themselves if it starts to bleed if they can easily reach it. Then there won't be any sign of injury just the blood that was left although you may on occassions notice one or more feathers in or around the cage that also has a bit of dried blood on it. Also at times when they are overpreening an area that is growing in new feathers they may go too deep and damage the skin underneath causing a small scab or red place beneath the feathers.
Next I would ask what you are feeding him. I know this will sound funny but we have actually had some people think something serious was wrong with their bird because they have dried "blood" on their feathers and it will later be found out that whatever the bird had eaten in the last 24 hours had a red coloration to it making the birds fecal matter have a blood like appearance. Of course if your bird only eats seed or fresh foods that had no reddish color this could not be the case.
If all else fails and you notice this problem on more than one occassion a vet checkup would be in order.
Good luck!