Hello, I'm Dr. Kara. I have over 20 years of experience as a veterinarian and I'd like to help with your concerns about your FIV positive kitty.
I don't think that you need to panic but I would recommend having your fellow examined by his veterinarian and have some blood testing done because of his FIV positive status. The dark color to his vomitus could be digested blood or bile, it may be difficult to tell just by looking at it.
Vomiting can be related to something as simple as a quick change in diet or treats, eating dry food too fast which leads to over eating, expansion in the stomach and vomiting, getting into something he should not have like the garbage, a bug, plant material or a toxin.
More serious causes are chronic pancreatitis, esophageal reflux, a dietary allergy or sensitivity, inflammatory bowel disease, heartworm disease, internal organ failure, hyperthyroidism, a full or partial gastrointestinal obstruction or even infiltrative cancers like lymphoma.
In his case even though he is a young cat because he's FIV positive I do worry about kidney disease or infections contributing to his stomach upset. Because these vomiting episodes have been recurrent if he isn't improving with my recommendations I would recommend an examination and checking some bloodwork to start, a complete blood count, biochemistry profile and T-4. Depending upon exam finding abdominal radiographs may be needed too. If those look normal then a heartworm test, as vomiting is strangely one of the most common signs of heartworm disease in cats, should be done as well as a specific blood test for pancreatitis, a fel spec PL (feline specific pancreatic lipase). This test is highly specific for pancreatitis in cats.
The next step diagnostically is an abdominal ultrasound and/or either exploratory surgery or endoscopy to gather biopsies of his gastrointestinal tract to look for inflammatory bowel disease or cancer.
To start now you can try giving him an acid reducer to see if that helps. You can try either:
1) Pepcid ac (famotodine) at 1/4 of a 10mg tablet per 5 to 10 pound cat every 12 to 24 hours
2) Prilosec (omeprazole) at 1/4 of a 10mg tablet per 5 to 10 pound cat every 24 hours.
These are very safe and can be used long term if necessary.
Dietary changes may help as well.
For now you can try a bland diet of 2/3 boiled minced white skinless chicken and 1/3 boiled white rice mixed with some low salt chicken broth to make it easy to lap up and swallow.
If this helps him there are some long term diet changes you can make that will help.
You could try feeding a canned only sensitive diet stomach. This will stop him from eating too much dry and having it expand in his stomach and also is easier on his stomach then regular foods. Royal Canin and Hills both make sensitive stomach foods. When you start to convert him from the bland diet to his new diet make sure to do so slowly, a little more new and less bland at each feeding. It should take 5 to 7 days to switch diets.
If he is still vomiting intermittently you can try more restrictive diets such as prescription Hills i/d or Purina Veterinary Diets EN. If these aren't enough then perhaps a true hypoallergenic diet such as Hills z/d or Royal Canin Duck and Green Pea will work.
If he continues vomiting, you notice dark, tarry stools, weight loss or lethargy then he absolutely must be seen by a veterinarian.
Best of luck with your fellow, please let me know if you have any further questions.