According to thesun.co.uk, sources at London's Moorfields Eye Hospital said she arrived wearing an eye-patch and said: "I can't see anything - I am totally blind."
They added: "She was clearly in a lot of pain."
Mel B, known as Scary Spice, 43, was transferred to Luton and Dunstable Hospital's ophthalmology department.
As soon as she could see again, she left, against doctor's orders, to rejoin the group rehearsing for the tour.
She will require medication and further intensive treatment, likely to include more hospital visits.
A source said: "Mel was in a really bad way. The other girls and their managers were really worried.
"Mel was frantic and screaming that she couldn't see a thing. She went to the hospital in a chauffeur-driven car with an assistant, who was holding her hand.
"No one knew what brought it on but she had very swift, and very good, treatment. It is unclear if the loss of vision is linked to her previous problems, although it does seem likely."
The Spice World 2019 tour will see the group, also consisting of Geri Horner, 46, Emma Bunton, 43, and Melanie Chisholm 45, - minus original member Victoria Beckham, 45 - belting out hits like "Wannabe", "2 become 1" and "Mama".