Never Make a Deal With a Dragon

Elder Dragon Form - Is very important, and should be gotten at levels 6,11,16

Yeah it is "highly reduced", but you should take this item against a hero that can outdps you in 1 v 1, or have a high burst. I've been recently playing against an Ursa, and Blade Mail killed him many times. Same goes for the spells ( Lina/ Lion/ etc). Same against a Huskar, I sometimes manage to cast it when he ults me and he died nearly instantly. It is situational and should be used against heroes with high burst or low life. Same against Sven under God's Strength But I can tell you that against Agi carries in late game it will most of the time have very high a damage output. And to me there is no "cost effective" on Blade Mail. It just returns the damage you take after reduction. In a 1v1 situation, it's really huge.

Getting Blademail on High Armor Heroes is not really cost effective, if you do the calculations you would see that the damage return is highly reduced.

Hum it's not a complete guide however the skill build is not bad in specific situations.

-> High auto attack harass
-> Sololaning

The point of getting Breathe Fire is to be able to react to ganks and have high enough a threat so people are careful fighting you. I do get the point of getting Dragon Blood till lvl 2 (even though I personnaly think lvl 1 is enough) but after that you'd better max Breathe Fire out. You will have a much better gank potential for the mid game.

Also, maxing dragon tail should happen before stats as the stun is quite long and good and will have much more benefits than 2 pts of stat. Yes 0.25s and 25dmg is not high but stuns are the most important and annoying, life saving, kill making skills of this game.

About the Urn of Shadows, even though it's nice I prefer it on ganking heroes. I mean 875gold for life regen as you are the hero with the highest regen thanks to your Dragon Blood is not really worth it espescially as you are that good a ganker. As you are not really mana intensive, claritys are enough for mana regen and items like Force Staff, Medallion of Courage or Blade Mail are better imho.