I think 'heretic' refers to anyone who doesn't mesh with the current pop theologies going on the time. I think one must be clear to distinguish 'heretic' from 'blaphemer' because they are, of course, very different.
There is much 'pop' theology today. The entire millennial/dispensational lens to Revelation is one of them. In the 80s, Frank Perretti's views on demons and angels created a similar lens. These lenses get crafted by 'pop' theologians/church members for better or for worse and everyone buys into them because people cannot think for themselves. Sometimes these lenses are positive, as in the case of Rick Warren's Purpose Driven Life and sometimes they are negative, as in the case of the entire "Left Behind" craze.
Anyways, a heretic, like me, is one who refuses to fit the popular mold of what a "Christian" must look like. Heretics are Christians, but they sometimes look differently, act differently, think differently and believe differently than "conventional wisdom". Examples of theological heretics would include people who do not let the "church" or pop Christianity give them a list of dogmatics to believe in. Some of this is for better, and some of it is for worse; but such may be:
--those who do not think the Bible is inerrant, those who believe in evolution, those who are pro choice, those who are pro gay, those who do not take miracles at face value, those who no longer can believe in some images of god, the irreligious, femminists, liberation theologies, preterists, etc. etc.
But, like I said, blasphemy is different. Heresy is an honest thing, people really do believe certain things, they really do call Jesus "Lord" and they really do believe in their hearts that God raised him (the "Biblical" formula for salvation). Some heretics may be saved, some may not be; but in either case it is not their 'heretical' nature that has influenced salvation.
However, blasphemy, like apostasy, has to do with a knowing, renuncianciation and turning away from KNOWN truth. In other words, someone really does believe that Jesus was resurrected, but they oppose it, they let the satan work against them inside them. This, I think, is what really messes people up. Finally, the former (heresy) usually refers to the minors in the faith while the later (blasphemy) tends to deal with the latter (such as the divinity of Christ, the historical resurrection, atonement and the forgiveness of sins).
There is a big difference between saying Christianity is true and not exactly agreeing with others on what that means and on knowing Christianity is, at least in some way, true and then saying it isn't.
Hope it helps.
PS, I am a member of the ELCA; Evangelical Lutheran (or Liberal) Church of America.
Thats, really ODD... 
