I bought a brand new copy of a WL'd paperback and in the middle of the book, on ONE page, is an inkblot (definitely a printer situation, since I bought it new last week and just took it out of the bag now and found the blot while reading.)

Is this now considered unpostable, or is it OK since the inkblot is on the page as a result of a printer glitch?

TIA!

Printer's blots are fine, but I'm curious as to how big this one is? Does it obscure several words, one word, part of a word, or nothing?

How big is the inkblot?  Does it interfere with reading?  I would still post it, but maybe that's not the "safest" thing to do.

You could post it to your shelf and PM the requestor about it before sending it.  Or you could offer it in the Book Bazaar for a credit.

Can you return it and get another 'clean' copy?

I would consider it postable

It's postable if the book is still readable. If it blocks a bunch of text and affects the reading of the book then I would try to exchange it.

I got 2 copies of the same book from Amazon once and both had pages that were bound backwords and upside down. I was able to read the book but I decided that it wasn't postable. I got a refund from Amazon. 

Most book stores or publishers will replace it for you.  If you decide to post it, it seems to me that it falls under the 'gray area' rule -- PM before sending.