This does seem to be getting off-topic, but, hey, why not?

I think that The Black Sleep had some "exploitation" success, as it was heavily promoted at the time. We're not talkin' The Ten Commandments or Ben-Hur here but respectable box-office, good bang for the buck for the producers. Also, its release more or less coincided with the Shock Theater package of old horrors being syndicated to local TV stations, and so it probably hung around in the theaters longer than usual for a black and white programmer.
Tom Weaver could probably tell us more (and I think Bill Warren has a chapter on it in KWTS), but just going by imdb THE BLACK SLEEP was filmed in February 1956 and released in June of that year. While Vampira's show aired public domain horrors (usually very short on the horror!) in Los Angeles as early as 1954, the Shock! package didn't start until October 1957. Imdb also states THE BLACK SLEEP was re-released in 1962.

My guess is that the picture didn't do all that much in 1956 (there doesn't seem to have been a call for a follow-up), and its re-release was probably a late attempt to ride the coattails of the Monster-boom that by that time was in full swing. (I'm not sure how successful the re-release would have been either, though. I think many kids may have shared my initial reaction and felt disappointed upon seeing so-many horror actors playing non-monster parts).

Not having even glanced at Castle's autobiography in ages I can't recall what inspired him to turn to horrors in the 50's. Macabre, I believe, predates even the Hammer horrors, so Castle was off and running before those bigger films hit, and Macabre made money.

I think Castle was more initially influenced by Hitchcock and his thrillers (even his TV show which started in 1955). Have you seen MACABRE? Its famous gimmick is the life insurance policy in case an audience member dies of fright watching it - which is a pretty safe bet since MACABRE isn't scary (it's not even a horror picture; it's a thriller about the kidnapping of a child).

HOUSE ON HAUNTED HILL - Castle's first real horror - comes after the debut of the Shock package (and the arrival of the Hammer gothics), but it also seems to take its inspiration from spook shows and 1950s horror comics.

Hammer's films are a different kettle of flesh, and I wonder if their success in the States was Shock Theater-related or was just gonna happen.
CURSE OF FRANKENSTEIN was produced and released before Shock Theater hit. Pretty much an outgrowth of their recent success with science fiction films that featured significant horror content joined with a proposal from America to make a Frankenstein film (from a script by pre-Amicus Milton Subotsky). The different look and feel to most Hammer horrors probably stems from them being aimed at adults rather than kids or teenagers (no floating skeletons or buzzing seats!).