The images of Osbourne, Tony Iommi, Geezer Butler and Bill Ward are being created in a 120ft (36.5m) mural.
Mr Murals said: “You meet celebrities and you never know quite how it’s gonna go, but you just forget that they’re just normal people.”
He was painting the Black Sabbath logo where the car pulled up, he added.
“But I think, because I’ve got masking tape all over it,…. [Sharon] was probably thinking, ‘What is going on with that bit?’. But she was still really kind about it.
“She… just loved the black and white stuff that’s on the wall.”