It happened on the stretch of the southbound carriageway just before junction 10 for Walsall, forcing police to close all four lanes, at around 11.30am.
Fire crews equipped with hose reels and breathing apparatus took around 40 minutes to control the blaze, which is thought to have started when sheets of polystyrene boards caught fire in the cab of a works van.
One lane of the motorway remained closed this afternoon while firefighters and workers from the Highways Agency continue to secure the area.
This afternoon Tipton-based firm Home Rescue Solutions Limited confirmed one of their vehicles was involved in the fire.
Company director Lee Walker said: "Two of our lads were in the van at the time and thankfully they are both fine."
One vehicle passenger tweeted the Express & Star with a picture of the delays.
Walsall FC goalkeeper Liam Roberts was among those who took to Twitter to report that some motorists were stopping to take photos of the scene.