Is slark that bad against blood seeker? I have only played that matchup once and I felt op as hell against him, just pounced into a forrest, if that wasn't an option I still felt like I could run around with his ult on me for days with the regen.
Bloodseeker is not as good of a counter to Slark anymore. The main issue is that BS is pretty good in the current patch/meta while Slark is close to garbage tier.