When is it advisable to take Blue cards in the first age in Seven Wonders?
A good question to ask yourself here: does everyone in my playgroup have the same distaste for blue cards that I feel?  If so, if they're predisposed to ignore them in favour of "cooler" cards, you could easily make a killing by snapping them up.

I will agree that something like the Pawnshop (3 victory points, that's it), doesn't seem like an exciting card, though it could make for a perfectly acceptable last or second-to-last pick: better than nothing right?  But stuff like the Altar or Theater, which are the first stages in the Altar-Temple-Pantheon and the Theater-Statue-Gardens chain, may be deceptively good.  2 victory points for a pick doesn't seem great.  12 or 11 points (respectively) for 3 picks is slightly better.  Are you playing a resource-light game where something like the Pantheon will be way out of your reach in Age III?  Then Altar could prove a much better pick for you than some other random nonsense.

Remember that every time you pick a resource, it costs you a pick.  Is picking up 3 resources better value for money than getting 12 easy points from the Altar, Temple and Pantheon chain?  You're already 12 points down, pick for pick, due to your concentration on resources, so you'd better hope you can make plenty of profit out of spending them...

I wouldn't want to praise the blue cards too highly: resources are the backbone of the game and you definitely don't want to end up in a situation where you have nothing to do in the final Age apart from discarding for coins!  But an experienced player who understands the chains, and which early picks can get you valuable later picks for free, can definitely make a tidy profit.  Just make sure the players with all the resources don't buy "your" upgrades out from under you to screw you over!