Although the IT Act of 2000 has no provision for blocking access to websites, DoT made it possible with Gazette Notification GSR 529 (E) - it said "Websites promoting hate content, slander or defamation of others, promoting gambling, promoting racism, violence and terrorism and other such material, in addition to promoting pornography, including child pornography, and violent sex can easily be blocked since all such websites may not claim constitutional right of free speech. Blocking of such websites may be equated to balanced flow of information and not censorship." But since this kind of block is issued directly by DoT and not through any Court, there is no public press notification or release.
The only way to know the reason of a block is with an RTI - which could take days if not weeks. What is easier though, is directly asking operators about the order from DoT, and possibly try to get a copy of the order - which is what we are in process of, you will know what's going on as soon as we do.
Update 21/7/11 #2: Reports are coming in that the issue has been completely resolved, and the sites mentioned above now open on Reliance, Airtel, MTNL and Fivenet – no other ISP had any type of complaint about any kind of ban. Airtel, Reliance as well as DoT has not commented anything on the issue yet.