• Is this normal?
• Has it gone bad?
• Does it still have the required properties?
• Why is that?

Yes, this is normal and does not impair the effect of the fertilizer.

A small quantity of iron(II) ions of the fertiliser oxidises to iron (III) due to the oxygen in air. Iron(III) ions and their complexes are generally coloured much more than Fe(II) ions.
The complex of preservatives and trivalent iron is very intensively coloured and leads to this brown-black solution.