Countering Bloodseeker is a matter of winning the first few levels before he can get his Bottle. The typical Bloodseeker build is QB+Stout Shield+Healing Salve. Assuming you're a ranged hero, trade as much damage as you can as soon as possible. If he ignores you and goes for last hits, you can pound him harder than he regenerates. After you've done this twice, he's out of regen and too low on health to come to the creep wave. If you're a melee hero, you'll have a bit harder of a time because you'll take a lot of creep damage doing this, but the same concept applies.
If you let him get an early Bottle or Boots or a few extra levels and extra points in Blood Bath, winning the lane becomes significantly harder. Instead, you need to take whatever damage he and the creeps deal to you and harass him for all it's worth, as you can be safe in the knowledge that he has no surprise spells to pull on you (other than a 120 damage DoT which just allows you to hit him harder).
Abuse creep aggro constantly. When the Bloodseeker goes in for last hits, draw the creep aggro on purpose by right clicking him and then backing off. This will cause his creeps to harass you and your creeps to chase those creeps. You can then drop the aggro, step in front of the wave, and deal extra harass damage if he goes for the last hit, or simply mess with his timing and then deny the creep. If he tries to trade with you, just micro well and this is exactly what you want because you have adequate regen where he's relying on a single Salve to last him until his Bottle. As a melee hero, Bloodseeker needs to have a health buffer to go for last hits, because you can kite him as he tries to run out of range and hit him 4-5 times if necessary.
If you're a melee hero, it's a bit different. You have to be equally aggressive in trading harass but you also have to expend mana (if you're a melee hero without a nuke you probably shouldn't be mid in the first place) to force him off the wave. This is actually usually really easy.
Bloodseeker is a very weak hero: his only strength is his Silence which can be useful against heroes that rely on mobility spells. In order to counter him, all you have to do is carry a TP Scroll. If he uses his ultimate on you and there's no tower close by to retreat to, you just TP away. He can't stop you, and has wasted a lot of time and effort ganking that he can't afford to as a carry.
Alternatively, you can just dual lane mid and he automatically loses the lane badly, usually even if an ally comes to make it a 2v2. Specific heroes can also beat Bloodseeker 1v1 without any real effort in solo mid simply because they're much stronger heroes or have strong slows to force him off the creep wave:
- Venomancer
- Viper
- Ancient Apparition
- Tiny
- Sand King (Level E)
- Templar Assassin
- Luna
- Stormspirit
- Lich
- Warlock
- Night Stalker
- Batrider
I'd link to a couple of replays but I literally haven't seen a Bloodseeker in weeks because the hero is just terrible once people learn to always carry TP's.
EDIT: I finally came across a Bloodseeker!
Best Answer
Yes- all damage types, potentially even HP removal, are redirected to their source. This definitely includes both physical and magical damage.