Are those two breeds the only way to make a blue cross bird?
There are plenty of chickens that could be used, and someone might have bred special lines that are not any breed we are familiar with.
Blue Sexlinks use the same sexing method that black sexlinks do (barred males, unbarred females.) So the mother needs to have barring, and the father needs to not have barring.
To have all the chicks be blue, one parent should be splash and the other parent black (=not blue or splash; does not have to be actual solid black.) So it could be a splash father and a black barred mother, or a black father and splash barred mother. The father could even be a Rhode Island Red (like so many other sexlinks have), and if the mother was splash barred you would get blue sexlinks.
As long as the parents have those particular color genes, any breed or mix will do.