I'm sorry you lost both kittens in such a horrible way. Had they ever been to the vets for routine vaccinations? It is critical to a kitten's health to get the kitten series of FVRCP. Panleukopenia can be tracked in on shoes, and is highly contagious, and deadly to unvaccinated animals. Another thought that crossed my mind is a highly virulant strain of calicivirus. http://www.petspourri.com:8080/veterinarian01_03.htm
The pinkish liquid from the black kitten's mouth sounds very much like pneumonia which could have been a symptom of a virus, and the kitten's inability to fight off infections. If you do a search (under user control panel at the top of the page here) and type in Emily Rose, I had a kitten some months back that had developed a horrible virus. Vaccinating for FVRCP did no good prior to the kitten's contracting it...it's a mutated strain and highly contagious. I did manage to save all four kittens from dying from complications of this virus, but it was a long and tiring time doing so. Emily was worst off with severe edema to her face and legs, but between antibiotics, and fluids, force feedings etc. she survived, and has been adopted out along with her 3 brothers. Better to be on the safe side in your case, and not take in any more kittens unless they've had at least two of the kitten vaccinations, and after you get an OK from your vet. Some viruses can remain in the environment for well over a year, and erradicating them is not an easy task.