If you put a darker colour over the top, in a few days the colour will fade from the blonde areas giving you a lighter shade of the dark colour IFYKWIM so you will have highlights but not in blonde
If you go to a hairdresser they will probably just put a toner through. (Thats what we used to do at the salon)

When I do my hair I go to a hairdressing wholesalers & get my stuff from them

The issue you will have by blonding over blonde, if you are using a strong product or peroxide is that the hair becomes weaker & weaker and will snap. Also if you are not careful you will end up with colour bands where you have overlapped the colour