Hello! Welcome to the hobby! I'm going to do my best to explain here:
1. Rams are known for "just dying".
2. Poor genetic makeup does weaken these fish.
1. Ram Cichlids are known for "just dying" and while it appears this way, it is much more complex than that. To put it into perspective - how many other animals can you think of that "just die" without any external stimuli? I personally can't think of any.
What is true, is that Ram Cichlids are very sensitive to incompatible water quality. Rams require warm temperatures (82-86 F), soft acidic water (low kH and gH, pH 7.0 or below), 0 ammonia, 0 nitrites, and as close to 0 as you can get with nitrates + frequent water changes. Some Black Ram (or Dark Knight) Ram breeders recommend doing 50% weekly or more. I have found great success doing 20-30% once a week.
If you can provide all of these pieces, you should have no issues.
If you do, it brings me to point #2 - genetics.
2. Genetics.
Dark Knight rams were developed in 2012 by a breeder in South Africa. Since then, a truly black strain (not a regular German with some dark characteristics) can be hard to find and if you do, it is probably a relative of only a few known breeding lines. To get the black color the original German Blue Ram was cultivated into the Gold Ram Cichlid and from them the Black Ram was developed. The sheer amount of inbreeding to get those beautiful colors has made this fish more likely to get sick etc. if their required parameters are not met.
Tried to make that short and sweet.
Can you provide me with:
Tank size
stocking?
EDITED: I saw that you had posted more information. 26C of 78F is too cool for rams. Unlikely that this alone killed them, but you'd need it to be 29C.