Defending blinds is a little tricky on regular tables. Inherently it implies seeing a flop out of position with a less than optimal hand. Most marginal hands like A9suited are much better either 3-bet (preferring a fold, but holding
equity), or simply folded out. Flat calling is really not the best play, and that's generally true for most hands.
But like everything else in poker it really comes down to who's raising your blind to begin with and where they are on the table. If a guy is doing 2xBB/3xBB raises every button or cutoff, you can probably re-raise broadways, medium aces, and medium pocket pairs with value in mind. Chances are your villain just isn't much stronger, possibly behind, and will fold to most c-bets post flop if they're loose raising. If you have a tighter player who's only raising their button once ever three or four rings, you may be better off just folding your question mark hands. Even though sometimes a TAG player will still be raising their button fairly light you just won't have much security in the hand. Sometimes you'll be folding the better hand with A9, but 78suited still has decent equity and position, and the TAG player is going to usually be tougher post flop. You don't have to beat everyone at the table - especially not the ones who are tough opponents when they have position!
On lower stakes, watch for the hair trigger shove guys when thinking of defending. The 3 betting rules don't apply because they 4 bet shove. Now, if I raised the pot with 77, as I often do, and someone 3 bet me, I'm really looking to fold or set mine. Those are the options most rational people consider. But there's a certain type of fish that looks at every raise as an attempt to bully them so they shove. I'd flat my playable range for these guys and only 3 bet things I'm willing to call a shove with (JJ+ for these guys). Here, don't bother with donk leads or check raising light to see a cheaper river or any other post flop defending strategies. These guys don't think, they just spew and over play their hands. Hit a two pair hand, a big draw with over cards, anything where you're likely to be ahead in the pot and make your money there.