I ended up with the smallest available "split system" heat pump which serves only the DR. There is no air communication to the outside (no light leaks), as it recycles room air (I have separate outside fresh air by negative pressure exhaust). It is very efficient and effective for both heating and cooling as well as dehumidifying (really, cooling). I cannot recommend these systems highly enough. They are used widely throughout the world and are beginning to replace forced air systems in the U.S. The unit cost for small units sufficient for small rooms is not much more than a stand alone cooling or window-type unit (actually the same as better units, ~$200-$400). The extra cost (either judged to be worth it or necessary) is in the installation, where it pays to obtain someone with experience in split systems.
Hi ROL, I'm in the process of designing my own darkroom and the whole ventilation system is the one I need more help with. Can you specify which split system you have?