Sorry. It’s deep-voiced radio actor Valentine Dyall, at least to anyone in the UK, of an age to remember him
(Obligatory Goon Show reference:
DYALL: … I am no longer the Man in Black, I am now the Man in Grey!
ANNOUNCER: What brought about this change?
DYALL: A very cheap dry cleaner.
ANNOUNCER: Very well, Mr. Dyall, the floor is yours … but remember - the ceiling is ours!
)
Johnny Cash? Pfui.
Dread Pirate Roberts.
I must say, I thought of Valentine Dyall first – but then I’m a smart-arse, and more inclined towards the Goon Show than country music. Johnny Cash is certainly a correct answer to the question as given, and more contemporary than Valentine Dyall, who hasn’t been the Man In Black since… what? The Fifties?
How was the question phrased? Was there some context that might have pointed you towards a 1950s British radio character?
Sure, it’s Johnny Cash and you were cheated but here’s an honourary mention for Jet Black (drummer with The Stranglers back when the old King was still alive).
Chaps of a certain age and type did sometimes refer to him as the ‘Man In Black’, cos he wore black. Like.
No doubt. Johnny Cash.
In God We Trust – All Others Pay Cash
WotNot: The question was phrased as I wrote it in the OP–there weren’t any clues given that it was a radio show host.
My first thought was Wesley - “with eyes like the sea after a storm.” 
Grim
I dont know if I should be revealing this here,but I am the one. 
Yup, it’s Johnny Cash. That’s a bit like asking “Who’s The King?” and expecting an answer besides Elvis.
Do people in England know Johnny Cash? Not much country music over there, despite the best efforts of the Rolling Stones.
Oh, even I, a dedicated classical music fan, have heard of Johnny Cash. But I don’t think of him as the man in black, merely a man in black. When I think of him at all.
Wow. Not many NASCAR fans here. My first thought was Johnny Cash, followed immediately by Dale Earnhardt. For an Earnhardt fan, that association is absolutely automatic.
I still say you should’ve said The Black Knight.
Arthur Pendragon?
Who’s the King of Rock and Roll?
Elvis
The Man in Black is Johnny Cash.
I have never even heard of Valentine Dyall…
What Airman said.
First reaction: Johnny Cash
Second reaction: The Dread Pirate Roberts.
Never heard of this Dyall fellow. Oh, I’m not into NASCAR either, so I had no idea that Dale Earnhardt is also known as TMIB.
Hmmmmm . . . well.
If you can accept that ‘Country’ music didn’t just appear on planet earth in a vacuum and that a label is just a label, maybe you can see that ‘country’ is folk music and that its roots lay quite some way – and a boat ride - to the east of the Appalachians and the Mississippi delta.
So yeah, England knows Johnny Cash, and Johnny Cash knew England (and Ireland).
But, of course, Johnny Cash was always more than a folk singer/writer/performer with an American twist. On a line from, say, Robert Johnson to the present day, there is one key period in time and place; that’s Sun Studios in the mid-late 50s. It’s probably no exaggeration to say four performers each contributed to the shape ‘popular’ music was to take for the decades to come; Carl Perkins, Elvis Presley, Jerry Lee Lewis and Johnny Cash.
Just as you might not always see an elderly relative in the face of grand kids, you know he or she is there somewhere in the character. And so it is so with Johnny Cash and modern popular music; his contribution to the essential character of what became rock and roll was a combination of that particular machismo quality, and a political/socially aware common (man) decency – for example, the standing up and taking no shit. There is as much Johnny Cash in the DNA of rock (and roll) as anyone else.
So again, yeah, England heard of Johnny Cash.
Of course “the ‘Man In Black’” is Johnny Cash. Accept no substitutes.
Johnny Cash. I think the radio show was just trying to be tricksy.