The husks themselves shouldn't interfere with ketosis as they're indigestible and just absorb liquid, bulking out and enabling you to 'go'.
Some unfortunate people who are hyper sensitive to carbs have reported coming out of ketosis when taking the psyllium husk capsules ... maybe there's something in the coating.
I have a teaspoon of PH in my shakes (hot or cold) and it thickens them up a bit ... not to everyone's taste but I love it. I've never suffered from constipation since taking PH regularly: I think I'd have thrown the towel in without them!
Word of warning though ... if you're thinking of taking PH, make sure it's just the pure husks with nothing added - SOME brands have digestive aids added (pro-biotics, pre-biotics etc) and they might have a negative effect on ketosis. Also make sure you drink an extra big glass of water after having them as that will enable them to do their work!