Bleach

Thankyou all so much. I think it will have to be a permanent marker, or a dylon pen.
This top was expensive, but it's not the cost so much,but the fact it's a favourite and I wear it alot.

I did that after accidentally buying a spray cleaner that had bleach in it. There was a bleached part on a navy blue favourite cardigan. I used a Sharpie permanent marker to cover it. It does fade over washing but it’s easy enough to re-do it.

Sorry to say that the brand Domestos was first produced in 1929 by Wilfrid Augustine Handley (1901–1975) [4] , a dentist from Heaton, Newcastle upon Tyne in the North East of England, Which is just over the river from me.

Fennel Domestos is a hydrogen chloride bleach, not the same as hydrogen peroxide, the laundry/hair bleach. The chloride one is fiercer.

You could get a dylon pen, £1.99 ebay. The colour does not fade and you only have to do it once.

As others have said I don't think there's any solution. That's why I won't have have Domestos in the house.
OTOH I once tried diluted hydrogen peroxide - used to bleach hair - to remove fishy/oily smells from boil in the bag kippers from some denims and it worked.
I don't know enough about chemistry to know if it's similar to Domestos.

Depending on the fabric, I reckon that permanent market pen would work, even though as Grannynannywanny says, it sounds hilarious. I've done that for tiny spots on my husband's new black T shirt caused by a splash of pool chemicals!

Again, according to what style, fabric etc., you could add some decoration to it, embroidery, a flower, something sparkly, or maybe some fabric paint (perhaps a marker pen would have the same effect). If you have to keep re-doing it, well doesn't matter really - better than not being able to wear it.

Yes, permanent black marker works well and on scuffed shoes.

Oh that’s so frustrating, did it myself years ago when I was cleaning my sink, a permanent marker might help but you will have to keep on doing it,

I’ve used permanent black marker ink before and it works well. I pressed over the mark using a cloth and hot iron which helped it to stay black longer.

You might get away with permanent black marker pen.
Isn’t it just typical though?
Like tomato sauce when you are wearing a white top.
The Law of Sod

Could you buy a small decorative item to stitch over it? I have tried the marker pen trick but it usually washes out or colour changes. Depends on the fabric I suspect.

I spattered a favourite comfy pair of black trousers with bleach last year. I was reluctant to part with them. I touch them up with black marker. When it fades in the wash I redo it. My grandkids think it’s hilarious.

I don’t know of any way to properly fix the problem.