According to this french article, French Blue will get the first batch of 350s originally ordered for Air Caraibes. This is why the employees of TX want to go on strike.
They are striking against the creation of the lower-cost French Blue, and the transfer of flights to that company. They see a trend to grow the new subsidiary and to kill the mother company. They are fighting for their professional prospects.
According to this article, the first two A350s will be delivered to Air Caraibes and the third one (I would think MSN5) will go to French Blue. TX's pilots were threatening to go on strike because flights to the Caribbeans normally operated by TX would have been transferred to French Blue.
I bet this latest twist is not the last one ... Groupe Dubreuil apparently wants a low-cost international brand, and most of its market is in the West Indies. Creating French Blue does not make sense if it is not serving the main market.