Is this part real LEGO or a knockoff?
I've shuffled through my pieces collection and found sadly none in dark blue, but all 17 I've found in either old grey or red have the same attributes you mention:

Top stud is hollow, with an H-shaped cutout
There is no LEGO logo or part number anywhere
There is a small seam that runs along the full length of the part on opposite sides
There is a small number (between 2 and 8) on the outside of the part on the bottom of the rounded section
The part feels like LEGO and has the new LEGO sheen

Single-digit number at the bottom:

H-shaped cutout in the hollow open stud:

Seam running over the opposite sides:

I'm 100% sure these are all genuine LEGO pieces, the red ones from 7419: Dragon Fortress.