Seems to me, a hero that wipes out and entire opposing team one after the other while standing toe to toe with multiple towers all while taking little damage is a bit absurd for a "weak" hero. 5 players, 2 towers little damage. I understand he siphons health after each kill but wtf.
And yes, I am very new to this.
It's important to understand, I think, that all power in dota is relative. It is not so much about how powerful each teams heroes are but a lot more about how fast you get powerful. Dota in its core is about gold and xp management, kills by themselves translate into that by the gold awarded to the killer plus the possible assists the gold loss on the killed and the downtime of waiting to respawn.
Something that I see often in my normal/high bracket games is a lack of understanding of when you are indeed stronger, when you should push and when you should farm. Seems that people get into this mindset that there is a way that their hero is supposed to be played and ignore the way the game needs to be played.
But I digress, my point is that, in that particular case Bloodseeker is really strong only because he got fed, he got ahead in xp and gold. When a hero gets ahead in xp and gold, especially any kind of carry, he is going to be dangerous, and in fact thats what playing a carry is all about: getting ahead. Not just getting stronger by farming/killing, its getting stronger FASTER by farming/killing.
Maybe that all sounds very basic, but I think that is where your problem lies, in the basics.
Just give it time, hang in there and keep playing.