I just went into "Wilkinsons" and found some stuff called "Tyre Shine" it's an own brand and costs £1.99 for a large can of it.
And I recommend it.. I used to armourall my tyres but this was costly cos it's more expensive and you dont get as much so I only use that on the interior now. But this Tyre Shine is good for blackening the tyre wall, its cheap and you get alot of it
Just thought I'd let you's know.