Can a third party file a patent based on a idea I have listed on my blog and made public under a creative commons license? Are improvements to the idea patentable if the original idea was published by me under a creative commons license?
This question might be better asked at avvo.com, where you could get in contact with a local intellectual property attorney.
The answer you're likely to get there, is that anyone can file a patent based on anything. Indeed, the third party may even be able to obtain a patent based on your idea, though there are procedures you can undertake in hopes of preventing that outcome.
Improvements to an idea typically are patentable, if the combination of the improvement with the original idea is "non-obvious." Relevant law is 35 USC 102-103, which you can find at law.cornell.edu. A big caveat is that if the idea is "abstract," i.e., not perceived as sufficiently "technological," then it (and the improvement) may not be patentable. 35 USC 101.
Creative commons license doesn't have any impact on patentability - it's your prior disclosure of the idea that limits whether a patent can be obtained. Whether CCL would affect enforceability of a patent based on the open-sourced idea, is a question sometimes kicked around but not yet definitively answered. The closest relevant law (that I know of) is Jacobsen v Katzer, which is well-described on wikipedia among many other places.