Exactly, Omega - they have no distinguishing marks.
They fly no nations' colours, they leave no message, they tell us nothing. Thus, we call them 'Outlaws'. If anything, it is more unusual to see a large fleet without markings than one with markings, and for there to be two attacks of largish fleets with no distinguishing features so soon to each other leads to the conclusion that they are linked.
We could point to the similarity in tactics - no demand to surrender or negotiations, just a strict formation. Also, although the ships are indeed 'hodge-podge', the fact that they're specifically using ships that belong to no one group in particular suggests a private backer with enough wealth to afford second-hand ships, not the newest warships off the line or private creations. All this points to some sort of outlaw or bandit organization, mercenaries in the hire of the Black Dragons.
Admittedly, if I said "It must be the Union! Omega Thrax in particular!" then I'd be way overstepping my bounds of knowledge, but suspecting the two attacks linked isn't too much. It's war. Things like this don't happen by chance.