I believe there are precious few questions relating to the concept in general, but I was wondering if the concept of an organism absorbing large amounts of CO2 from blood seemed reasonably plausible? This question also could relate to the plausibility of nanorobots (which could be powered by CO2 in theory). What if such a parasitic creature was able to form some structure, like a mini organ that help it suck up CO2?
Strictly speaking such an organism would be a symbiont and not a parasite.
Unfortunately CO2 is quite low in energy so using it for "fuel" (either by an organism or by nanorobots) is problematic.
Plants actually use CO2, but they need to add energy to it in order to produce sugar+oxygen (plants actually breathe Oxygen and produce CO2, but photosynthesis is predominant, at least in daytime).
If You want to speculate about such a symbiote then You can think about something residing on (or in) the skin and using light to produce Oxygen from Carbon Dioxide and, perhaps also shielding from harmful radiation (ultraviolet and up).
Here are the variables:
CO2 is not an energy source: As ZeoByte mentioned CO2 cannot be used for energy, this is why plants need sunlight. They convert the easily found CO2 into sugar which is an energy source they burn at night or when they need to.
Parasite or Symbiote?: Parasites harm their hosts usually by taking something they need without giving anything of equivalence in return or cause collateral damage to their host. Symbiotes form a mutually beneficial relationship with their host. Example: a clown fish living in a anemone. The clown fish gets protection while the anemone gets bait to lure in potential food, that's a symbiotic relationship.
With that out of the way we can focus on the mechanics:
Could a symbiote invade the human body and extract CO2 from the blood? yes;
The immune system: The biggest challenge in this is the immune system which would react to kill any foreign organism or end up killing the host in overreaction (ie a fever > 108 degrees). Though, there are plenty of parasytic adaptations evolved to prevent this from happening.
The organism: Needs a way to control its growth, if it grows in the wrong place it could impede important biological functions [constricting the heart, causing strokes or heart attacks etc.].
The Energy: The organism would need to get energy in order to convert the CO2. It could get this from sunlight on the skin or it could get a fair amount from body heat. Note: it wouldn't necessarily turn skin green, not all photosynthetic organisms are green.
Could it form its own organ in the host? : yes and no, It would need evolutional help from the host to truly develop an efficient symbiotic organelle. Fortunately, our own intestines are a prime example of this very concept. Could it create one? sort of, it could evolutionarily know where to burrow in the body so as to not spread or grow into vital spots. There are parasites that avoid aggressive intrusion however there are plenty that love to explore.
Finally Nanorobots: No, this is madness. Just as plants can't be powered by CO2 nor could nanorobots. Nanorobots could be solar powered like plants but will never be CO2 powered. Nanorobots could extract the "C"arbon from CO2 to make strong carbon structures but that would be the extent of their efficient O2 production.
And no there arent few questions on this subject. Plenty of scientists are exploring this and hundreds of other angles on this. It wouldn't surprise me if in 20-100 years we have biological or synthetic nano-micro machine capable of building structures from air and sunlight. Well maybe 200.
If your parasite is coming from the outside, there's a big fat chance that it will trigger immune system. Best chance that its cells come from modifying existing somatic cell (probably skin is the best, like ZioByte answer) into a mini-organ (a green swelling in the skin, with mutated chlorophyll).
Maybe in your world there is not enough sunlight to supply the energy needed for photosynthesis, so this plant evolved to live inside its host, taking the heat to perform photosynthesis.
The problem is, because heat is available 24/7, then it proceeds to absorb body heat until the host suffer from hypothermia, and finally die.
This is not really plausible, but may provide a way how this plant can be parasitic although it consumes CO2 (which is beneficial).