Fire crews were called out to Lower Horseley Fields at around 9pm on Monday, which led to a number of roads being closed and railway lines being shut as firefighters tackled the flames.
A string of active businesses were engulfed in the blaze, which measured approximately 200m x 200m, with owners speaking of the disruption it has caused to their livelihoods in the run-up to Christmas.
The fire was finally extinguished on Thursday after days of round-the-clock work by crews from across the West Midlands.
Fire investigators initially looked into the cause and said they believe the fire was started deliberately.
West Midlands Police has now confirmed it has launched an investigation and anyone with information is urged to come forward.
A spokesman said: "We are treating the fire as arson.
"Anyone with any information is urged to contact live chat via our website quoting crime reference number 20/1014469/22."
A number of roads near the scene of the fire remain closed. Willenhall Road has now reopened to traffic heading into the city with a signposted diversion for outbound traffic - whicg has to travel along Ring Road, Bilston Road and Stowheath Lane - in place.
Lower Walsall Street remains closed until further notice, as does the canal and towpath between Horseley Fields Bridge & Walsall Street.
At its height 28 fire engines and more than 100 firefighters were on the scene. With police roadblocks stopping traffic passing through the industrial estate yesterday, businesses around the area began to feel the effect of diverted traffic and late deliveries.
Steve Phillips, the owner of Interplumb on Willenhall Road, talked about the lack of customers. He said: “Normally we have a good few customers by now, but I’ve hardly had a customer all day.”
“The road blockage is adding around 10 to 20 minutes in rush hour, the ‘tradies’ don’t have that time on their hands, so they go somewhere that is more convenient and faster for them.”
Mr Phillips added: “It has affected my bottom line for the last few days, I am supposed to break up Friday but we shall see.”Debris from the fire has been thrown around the area, with a lot of it landing on gardens, forecourts and building yards.
One York Street resident who lives within a few streets of the fire talked about how her garden is “littered” with fallen debris and possibly asbestos.
She said: “We had so much fallout and smoke from the fire, I know my garden is full of ash and bits of the roof from when it was going on.
“We don’t know what it was, but it looks like asbestos so we don’t want to touch it. If it’s asbestos we don’t want to mess with it.”
A staff member from Wolverhampton Signs, on Commercial Road, said: “We haven’t really had any issues with business so far, but we have had a bit of debris fall around.
“Some of the looks like asbestos, we don’t want to mess with it as it can be quite dangerous.”
The investigation into the cause of the fire continues.