Where and when do the blue whales mate?

Any animal can physically mate with another animal of a different species, but two animals of different species cannot reproduce. The definition of a species is 'a taxonomic group whose members can interbreed'. There are some exceptions (e.g. Horses and donkeys, which can interbreed to produce mules), but whales are not one of them.

While blue whales and orcas are both whales, they are two different species of animal, therefore they cannot reproduce.

By sheer difference in size if nothing else this would be impossible. BUT Killer whales or Orcas are not whales but actually Dolphins.