I'd say a Blink Dagger, or Shadow Blade for that matter, is situational. If you don't have teammates with disables or slows to help you close the gap on an enemy hero. Core items for Legion Commander should either focus on improving durability or attack speed. Power Treads > Armlet > Assault Cuirass is legit but I'd argue for a Basher over a Deso. Even if you don't turn it into an Abyssal. Her problem, if you're snowballing, isn't damage. Her problem, kinda like Bloodseeker, is locking people down when you don't have an ult. But by the time you farm up to AC, you may already have so much damage from duels that a disable doesn't matter.
Deso seems out of place on Legion Commander whereas a Basher can be upgraded to give you more damage along with the disable.