Is it "common" for gals to discount references based on the said references ad choices? ...I don't think it's common, but I'm sure it happens.

Do you need to consider a gal's ad choices in your screening criteria?  ...Not in my opinion, as long as you are using p411 and checking reviews, you should be good to go.  A lot of gals advertise via several outlets, including BP.  

Are you running any personal risk seeing ladies who advertise on BP? ...again, not if you are using P411 and checking reviews.  Just be aware that BP is a just an ad site, with few, if any rules... You'll find a lot of fake and fraudulent ads, scams, etc., but there are legit ads too.  

Disclaimer... There is always some risk, but you minimize it by using a verification site and checking reviews.

in passing on you just because your references have used BP, among other places, to advertise.

Some of the best providers in my area advertise on BP. Most also advertise on P411 or other places. But to dismiss a reference only because they've used BP, when they are also on P411 and have many very good reviews on TER, seems unreasonable to me.  But, the screening process is the provider's prerogative.  

I've provided references who have used BP and have never had any problems with my references being accepted. (These references also had a review history on TER.)

both also advertised on P411 should have negated any unsavory associations with backpage. I guess it can feel icky for some, especially for ladies from places where there are many viable advertising venues. To those ladies, BP seems like a kind of El Cheapo last-resort place to advertise. But for ladies from smaller areas with very few advertising options, BP may indeed be the most viable, and would not have the same negative association.  

So if your visiting lady was from a place where there are many advertising opportunities, she may have a negative view of BP. Maybe find a lady who isn't on BP just to add to your list of possible references, or simply move on as another lady suggested. It's sad that BP has such a reputation, as many gents find their feet in the hobby there (probably because of how much it is in the news- great advertising!), but it is what it is. Good luck!!

I couldn't tell if this was more of a "one-off" or "the norm".  Am thinking it's more toward the former.