The premise of "Blind Date" is that Jack sets Liz up on a date with his friend "Thomas." Long story short: Liz arrives at the restaurant for the date only to be greeted by the beautiful Gretchen Thomas, a woman, portrayed by Stephanie March (who many lesbian, bisexual, and queer women also lurve for her portrayal of ADA Alex Cabot and subtextual relationship with Olivia Benson).
Anyway, during the date, the following conversation ensues:
Liz: [laughing] Why would Jack just assume that we're lesbians?!
Gretchen: I am a lesbian.
Upon hearing this revelation, Liz guffaws a dorky, overly-enthusiastic: "That's awesome!"
The humor is in how the supposedly open-minded Liz clearly had a stereotypical image in her head of what lesbians look like and Gretchen, being blonde, feminine, and beautiful, did not fit into that image. This subversion of stereotypes is reinforced when Gretchen asks Liz if people mistake her for a lesbian often and Liz responds by playing a montage in her head of people mistaking her younger self for a lesbian and a boy. The message: The hetero Liz Lemon out-lesbians actual lesbians. Er, that is, lesbians can be stereotypically feminine and heterosexual women can be stereotypically masculine. (Yeah, not news to many of us, but I suppose we give mainstream television brownie points for this observation).
Secondly, during the episode, Gretchen and Liz hit it off and discover that they are compatible in their own nerdy, if not sapphic, way. They talk about plastics, joke about being like "Oprah and Gayle," and bond over being single (Gretchen on being scared of dying alone in her apartment: "And it’s not just choking, ever since I turned thirty every time I get in or out of the bathtub I think in my head 'Careful…careful…'"). In this way is a (albeit fleeting) lesbian character not presented as Other, but as akin to, and perhaps even more "normal" than, Liz's heteronormative main character.
Indeed, the audience is invited to be somewhat appalled at Liz's use of Gretchen. Although both women are aware of the other's sexual orientation, their continual "dates" blur the line between platonic friendship and romance. Gretchen at first expressed no interest in "chasing a straight girl," but Liz, bored, lonely, and flattered that Gretchen thinks she's "great," pursues an ambiguous relationship by initiating further contact, date-like encounters, and trips to IKEA. When Gretchen expresses concern about their evolving girlfriend-like relationship and says they need to take a break, Liz responds:
Liz: Okay wait, what if we made a pact. What if we say that in like 25 years if neither of us has someone, we'll move in together and be roommates? And even though I'm not into the sex stuff, if it helps you... I would let you... [looking grossed out] do stuff... to me.
Gretchen: [not amused] I can't be around you anymore. Bye Liz.
So, in answer to Bendix's question. I give 30 Rock a pass because the joke's not on lesbians. But, it's not on heterosexual women either. It's more specific than that and that's why it's funny. It's on clueless people, like Liz Lemon, who think they're liberal or progressive yet who also don't understand why a lesbian wouldn't leap at the chance to be in a pseudo-relationship with a straight woman who is grossed out by the thought of "doing stuff" with another woman.
This is not to say that 30 Rock always gets the humor right. Just that, in this episode, I believe it did.