BBs fruit makes Luffy literally not rubber, neither seastone nor haki does that.

Have we seen Blackbeard void a Zoan?

Plus seastone doesn't make you a normal human. Luffy is still rubber, he's just weaker with it on, we saw it in Udon.

Like, you put Seastone on Boa, and she won't be able to use her power. Likewise, we saw a Seastone cage stop Smoker from escaping, implying he can't turn into smoke, achieving the same core thing as Blackbeard grabbing them. We've also seen it remove Orochi's ability to transform outright when a Seastone nail was used on him.

It has also been implied that quality, and quantity, have an impact on how well it works. But, that being said, we've also seen Haki has power thresholds as well. Reasonably, the Darkness fruit would as well, since we've seen fruit effects countered by similar mechanics.

But, at the end of the day, all this stuff ultimately works however Oda needs it to work.