In As You Like It, Act 4, Sc. 1, lines 34-35, Jaques says "blank verse." What does he mean when he says this?Does it refer any how to Orlando's previous conversation with Jaques?
Yes, he is probably referring to the conversation that he had previous with Orlando in Act III, scene ii, in which Jaques puts Orlando down for being in love. Jaques is a bitter sort of chap, who doesn't see much value in any of life and always has a witty and deprecating remark for someone. He considers Orlando to be completely off his rocker, a fool, for falling in love. In essence, for Jaques, falling in love is for chumps.
So, though the whole of Jaques' conversation with Rosalind in Act IV, scene i has been in prose and not verse (prose is just regular sentences, while verse is written in iambic pentameter as poetry), he alludes to the fact that, once Orlando enters, the two will commence speaking in blank verse. It is called blank verse simply because it does not depend upon any sort of rhyme scheme. The lines are iambic pentameter, but not necessarily with a rhyme scheme at the end of the lines.
Verse was a common choice for Shakespeare between high-born characters in love. He used verse for important, dignified or heightened moments in a play. His lovers often use a lot of imagery (like Romeo) and these poetic phrases were written in verse. Here, however, Jaques is wrong. Orlando and Rosalind do continue their "fake" courtship, but they speak in prose not blank verse. Shakespeare shows that they are speaking honestly and simply together, just being themselves rather than "lovers" by choosing to have them speak in prose rather than verse.