you can just watch all their scenes on youtube if you want! if you just search dan/blair season 1 or whatever number the fangirls have already compiled most things. i mean, you probably have to watch season one to get the broader show context of everything but i stopped watching in s2 and i can gather the relevant information from the youtube clips. but i've also been relatively up to date on it because of tumblr so i don't know! personally, i'm maybe hating myself enough to watch the whole thing from the beginning, which means i don't really know how to advise you, but i would say that dan/blair actually kicks off in season four, so if you wikipedia the first three seasons you should be just fine.

that was a really long answer i'm sorry.