I've been thinking about it. You need blood willingly given or disacarded and Dumbly has a sweet tooth. He may get blisters that pop when he brushes his tetth and that toothbrush could have Dumbly's blood on it. Just put a bit of water on the toothbrush and then get the drops of blood and salivia that the toothbrush will diespense after getting wet once more.
Hope that helps a bit.
Author's response
Heya Disruptor.
There are gonna be problems however I do this. Dumbledore is far too paranoid, especially now to not recognize the hazards or potential hazards of blood, so, I cannot see him not scouring everything of any lost blood. that's why I set up his 'accident' in August.
Because he was knocked out("We found you at the bottom of a flight of stairs, unconscious and bleeding heavily.") there's no way to account for any missing blood and Dumbledore will assume the house elves cleaned up the mess. They can verify that, because by the time they got there Dobby had already gone. They cleaned up the blood /that they found/.
After much brain-wracking, I've decided to go with the 'military necessity' angle. For the nanites to be effective, Dumbley had to ingest a certain amount, Harry had to ensure he couldn't detect any difference in his lemon drops and so, he used the compulsions. That he used the blood to anchor them was the spur of the moment decision. the blood was on hand, so he used it. That decision will give him a few days of heartburn in the next chapter but it will be worked out, by someone with a strong will.
Alorkin