and if someone searches for "blue widget" and you are targeting [blue widget] your ad will be invoked.
are you saying that you want to attract searchers of: "blue widget" but NOT of: blue widget
(i.e. Non-standard and non standard are the same word, but non/standard is considered a different word - been a while since I've tested the above, so that example might be off).
Last time I tested, " & ' invoked different SERP results, but not different AdWords ads (even if it's a single keyword). Therefore, I don't think you can advertise to someone only using quotes.