Introduction:

Bloodseeker is an underused and underrated hero. The main purpose of this guide will be to change the view on this hero by changing the way dota gamers play him.

You might be of the idea that you got better heroes to pick than bloodseeker, but throughout this guide I will try to show you a completely new and smarter bloodseeker.

If you are new to the game and want to learn a new hero, in this case bloodseeker, I suggest you to read the hero page description:
Introduction to bloodseeker
and then play a few games with him.

Why bloodseeker?
Pros:

Utterly rapes public games
Very intuitive to play
The better you become, the more you rape
Has a silence
Quick gameplay, much action
Very versatile in both item- and skillbuild
You don't have to return to base
Easy laning
High mobility

Cons:

Melee
Does not push or defend towers well
Long casting animation (even if you cancel the backswing, it still takes alot of time to cast a spell)
Bloodbath does not work on illusions
Bloodrage is a double-edged sword; 1. silence your enemy, but increase his damage or 2. increase your damage, but silence yourself
Does not have a natural stun, disable or burst damage

Starting items:

+
Inventory:

Reasoning:
Buy a chicken and upgrade it later to crow. This will allow you to never return to base and also prepare you for more organized games where you always got a chicken.
Chicken allows you to stomp your opponent in lane. The difference between a good solo mid player and a bad one is often differentiated on three concepts:

How to deal with ganks; both from enemies and from allies. Stay uphill on your ramp and only go for a creep if you do not put yourself into a bad position. The most important aspect of laning is staying alive and getting experience, having creep control is second priority.

Controlling runes; crow allow you to take runes without leaving the lane. Bloodseeker can get a bottle, but can work well without it. Let your allies help securing runes

Getting items; some solo mid heroes, i.e. shadow fiend or tinker, need to get bottle asap to dominate the lane and without chicken this is not possible. Others needs boots, i.e. batrider or lion. Bloodseeker also needs stick, stoutshield, boots and other items as soon as possible depending on your opponent

the GG branches and regen is simply gotten to get some starting stats and some regen.
Regeneration items are very important on bloodseeker.
Most solo mid heroes are ranged and are able to harrass Bloodseeker alot. Bloodbath and an early stoutshield/pms can help against that and quelling blade can allow you to move towards creeps earlier.
Sometimes you do a mistake, your opponent got nukes, you get ganked or you don't get the creep you wanted to lasthit/deny, which puts you at low health. Without any regen items, you can't go to creeps without risking to die to nukes or harrash, but with regen items, you secure yourself from alot of nasty situations that might happen.
Higher levels of bloodbath is also taken to protect yourself from harrash, but with a few regen items, you allow yourself to get thirst earlier, if your early laning went well.

Bonus Info: If you play bloodseeker in a side lane, most of the above applies aswell, however the most basic items are available from the sideshop, so getting a stoutshield, quelling blade or more stats items instead of a chicken is worth it. Later on you can buy a crow, when the need arises.

Public Replay:
Bloodseeker in a public -AP game

Skill build

Info:
If your team wants to go for firstblood level 1, you should skill bloodrage as your first skill. The damage over time is great and the silence can also work wonders against some heroes.
You generally need 1 level of bloodbath, but their might be some games with lots of action, where skilling bloodbath at level 4 is the best choice.
Skill thirst as early as possible. The earlier you get it, the less team dependent you become and the more mobility you get.
However laning is far more important, so always try to balance it between how much health-regain from bloodbath you need and how much health-regain you can sacrifice without losing lane control.

Your item choice

This is the main reason why bloodseeker is not a common pick in competitive dota. His biggest disadvantage is that he is melee, and his best advantage is that he can potentially go down to 1 hp and survive the fight with full hp.
The advantage for ranged heroes is that they have an easier time dealing damage than melee heroes, because they can position themselves better and safer.
You probably often heard that Bloodseeker is an imba ganker, but he lose his effectiveness in teamfights. If you ever played bloodseeker, you probably also experienced completely raping the game and then getting completely useless as soon as your opponents start to go as 5?

This is only true, because you went the wrong items and because you initiate incorrectly with bloodseeker. I will now tell you the secrets that will unlock the potential to first rape your lane, then rape the other lanes, and then rape your opponent team in teamfights.

I recommend getting either:

Power Treads:
The PT option is taken for the IAS and the +8 stats-switching it allows.
Ever heard about PT-bottle-abuse? You put PT-Agi, then use bottle, then change to either PT-Int or PT-Str. This way you get more regen from each tick from your bottle.
The same can be applied to bloodseeker. Before your finishing blow, you click on PT-Str so they turn into PT-Int, while killing the hero. After killing the hero you double click, and you recieved a larger amount of HP this way.

You get this item if you need to be strong and a front-to-front autoattack hero.

Phase boots:
Phase boots allow you to move between lanes faster. They allow you to get into position to cast your spell before, your enemy can turn around - even in daylight. Phase boots allow you to charge up and get the last hit, that tricks the thirst ability. Phase boots allow you to ignore creeps and makes you able to dance around heroes, allies and enemies alike.

You get this item if you want mobility, unpredictability and a less confronting bloodseeker.

Info: If you want to take your bloodseeker to the next level, start using phaseboots. After a couple of games, you will not regret it. Without a hotkey tool for inventory, it might not be that effective though.