40% damage buff to teammate's spells, since it no longer silences and also amps spell damage. Crazy 6.82 update.
This includes amplifying ultimate damage.

Hero: ultimate damage / ultimate damage with Aghanim's Scepter / ultimate damage with BS's 140% amp damage without Aghs

Crystal Maiden in fact. Normal: 250dps; Aghs: 310dps; 140%: 350dps

Lion: 850 / 1025 (aghs) / 1190 (140%)

NP: 225 / 275 (aghs) / 315 (140%)

OD: I may be reading it wrong, but I think Aghs is just a +10% bonus.

QoP: 600 / 725 (aghs) / 840 (140%)

SK: +20%?

Veno: 81 / 108 (aghs) / 113 (140%)

WD: 120 / 150 (aghs) / 168 (140%)

Zeus: 475 / 640 (aghs) / 665 (140%)

So essentailly BS is a free Aghs for any of the above team members. They can skip Aghs and go right for their next item. In the case of Zeus, a first item Refresher Orb with a Bloodseeker teammate is 4200 gold faster now.