True, except that X. violaceum is used specifically to designate the dark stemmed. glaucous leaved variety of X. sagittifolium known as 'Blue Tannia', 'Black Stem Malanga' etc as opposed to the standard green form that has green stems and plain green leaves. The leaf blade form is also slightly different between the two.
X. violaceum

X. sagittifolium green form
There's also X. Sag 'Lime Zinger'