Sort of, but I can see her reasoning. Blair played a mojor part in the whole Jack thing, where as Jenny was all Chucks fault. Ontop of this, Blair really hates Jenny, so she sees it as especially bad.
I wouldn't use the word 'selfish', because both things really hurt her, but it made me question her character, I mean getting more upset about your ex sleeping with someone, than over your boyfriend whoring you out to his (rapist-)uncle? I just don't get that.
I'm still not sure from where her Jenny hate came from, they were fine until midseason (even got along well in the season 2 final) so to me it seemed the writers just destroyed another femalerelationship to have them fight over a boy...
they will get over it with TIME
after they hook-up with other people they will realize that they belong together
and they can't fight it (or at least that's my opinion)
I think Chuck and Blair ruley belong togeather, so I really hope they will be able to work past this and eventually come to realise that they were made for each other.
I think with some time apart they will both realise how much they need each other (I think other people find them hard to understand in a relationship) and get back togeather.