Is curry ketchup just ketchup with curry powder?
I suppose the Belgian "curry ketchup" is similar to the German varieties and no, they are usually not only seasoned with curry powder, but also other spices. There is no standard seasoning mix for curry ketchup, but common additional spices are ginger, black pepper, paprika, cayenne or other chili powders. I also think that some brands contain onions.

The brownisher colour of curry ketchup is mostly because of the turmeric in the curry powder, so just adding the powder to regular ketchup will get you a colour match even without the additional spices or ingredients.