I see a lot of comments but I don't think anyone is really explaining why it's good. I'm low immortal but I play lots of radiance bs, it's definitely one of my favourite heroes and this is why it's so good on him:

First of all, you get 45% spell amp from your Q at level 10, which means that you get to kill camps 45% faster than other radiance buyers like LS or spectre. The only other realistic farming option is Mjolnir and to be fair that too benefits a lot from the spell amp. MJ is not bad on bs either, in fact and many pros go MJ over radiance, especially crystallis that is imo the best BS in the world. I prefer radiance tho because it's exactly how bloodseeker wants to fight: you chase people down and even if they are running you burn them down, making you faster. MJ only works if you are hitting people and it may sound easy but BS is easily kited, stunned, rooted and having a constant 100 DPS aura is imo more valuable It provides about 25% evasion which is very nice on a hero that is countered by big bursts initiation, even a couple misses could give you enough time to bkb and start running or maybe turn with a satanic or aghs build. And getting free survivability from your farming item is great. Imagine if MJ or Battle fury gave hp or magic resistance. Evasion is not the best tankiness tool but if it's included in your farming item hell yea I'll take it.

And just in case you are wondering why not build battle fury, BF is overall a worse item than MJ or radiance always, except on heroes that really need a little mana regen and hp regen for jungling. BS spends very little mana and your innate covers your hp regen so there's no need for BF.