Hmmh… as far as I can tell that looks quite normal non-hemolytic growth typical to Bacillus species, though the angle and light are not ideal for spotting it. I am not sure what causes the darker coloration of the agar (I’d call it deep red rather than black from what I see), but it is a common phenomenon possibly related to the use of other nutrients in the medium and to the change of pH or something. Bacteria break down and secrete various compounds, so I would not be surprised if some of those cause the darkening of the agar. Admittedly, I do not know the exact answer so if someone does, let us know. And the dark spot seems to be created by the administration of bacteriophages, as indicated in the text.

The answer for the second question depends on the hemolysin in question, but most are expressed only in specific conditions. For example, the presence of iron inactivates some types of hemolysin expression in bacteria that use it to obtain iron from red blood cells. Thus, on standard blood agar these bacteria (e.g. Streptococcus pyogenes) virtually always cause hemolysis, since it does not contain free iron. Some other hemolysins are targeted against other cell types (e.g. leukocytes) and their expression patterns are different and related to the presence of these cells.

Sorry, I don’t have any specific references at hand currently.