1400 gold for 100 hp bonus hp from them. That's about 260 gold extra efficiency. Not only is that completely insignificant (akin to saying Frozen Heart is good cuz lol gives 100 hp), but it's also his fucking ENTIRE PASSIVE! That's a garbage passive! He's sacrificing an entire passive slot to get, at most, 300 extra health from items if he actually built several mana items or a tear. The 100 health is not even an upgrade on him compared to, say, a damaging passive that helps him scale the AP the seals give even more (50 extra damage in a rotation, for example, would be far superior to that 100 health).
So, no, if we look at the actual context in which Singed's kit synergizes with multiple dark seals, the 100 health is completely irrelevant. It's entirely rooted in the fact that he backs often to get lots of corrupting potions.