The priority on the move is intentionally lower than that of other shadow moves, as is regular trialblazer. Normally the game uses a priority system to gauge when two red attack boxes collide together to determine a winner. The priority usually goes light>medium>heavy>special>shadow>unblockable, with all air moves decreasing priority by a single point (unblockables being the exception).
If you were to test it, the red attack box of Shadow trailblazer will lose to almost anything, while something like shadow cold shoulder versus shadow wind kick will result in a possible trade or draw. It being throwable is also a weakness, but you just got to be smart about using it. On the plus side, it does travel fairly far, and is a whopping +4 on block, which allows you to continue pressure if you aren’t shadow countered.
Also, Cinder’s light DP does have invincibility frames, but because it’s an airborne move technically it loses in a number of situations you wouldn’t expect it too (decreased priority), and I mean flat out lose. I’ve been cold shouldered out of it sometimes. Not a trade, a flat out loss against it, because Glacius’ cold shoulder is a grounded special, and Cinder’s DP is technically aerial, lowering it’s priority to that of a heavy.