The Mature Blue Dragon Sinew is the tendon of dragonkin from the blue dragonflight. It can be taken from Azuregos in Azshara and dragonkin in Winterspring.
DOES NOT drop from the Cobalt Broodlings in Winterspring.
It does not Bind on Pickup, so you can buy it from someone else or from the auction house.
Patch changes[]
Patch 1.5.0 (patch date:2005-06-07): Mature Blue Dragon Sinew can both now drop off appropriate level blue dragonkin monsters. It should be noted that Azuregos will guarantee a drop but to get it from the lesser creatures will be a much rarer occurrence.
[60R] Ancient Sinew Wrapped Lamina