Let us talk about homicidal poisons first. A bitter poison, like strychnine, can obviously not be given with homicidal intent. The victim would immediately spit it out. Thus a homicidal poison must be tasteless. Similarly a poison which is colored can not be given.
Copper sulphate is poisonous, but it is blue colored.
If a killer mixes it in water or milk, it will render these liquids blue, and the victim would not drink it. Thus a homicidal poison should be colorless too. There are several other criteria which have to be fulfilled as well before a poison qualifies as an ideal homicidal poison. Because then the public at large would think that the person died of that natural disease whose symptoms the poisoning mimics, and the killer would go scot free. Arsenic qualifies well in all the above criteria.