A friend has an innovative idea about a service. To protect the idea from future patents, he has the idea to make the invention available to the public domain.
Is a blog a relevant place to describe the invention? Should he write an article in a specialized magazine as well?
How risky it is to make that idea available for everyone?
I'm with with Ross. A provisional patent is actually better than posting on a blog because it offers you protection AND you don't make it publicly available. From a practical IP point of view, keeping the idea secret for as long as possible is the key to actual protection more so than any form of publication ever could.
Also the Board of Patent Appeals and Interferences held that U.S. provisional patent applications act as prior art under 35 U.S.C. §102(e) as of their filing date. This is assuming you file for and are granted the subsequent non-provisional patent based on the provisional.
That said, your friend needs to first figure out whether his "idea" for a business service is actually patentable. See: http://www.legalzoom.com/intellectual-property-rights/patents/what-kind-business-models Also see the recent SCOTUS Bilski decision: http://en.wikipedia.org/wiki/In_re_Bilski
1) A patent is only as good as the amount of money you have readily available to defend it.
2) Patenting an "idea" might actually attract unwanted attention to your "special idea". In the case of unwanted attention refer to #1 above.