No big thing. They probably had you and multiple other parties sign such contracts while they were determining the right person to hire. The fact that you did sign a blank contract causes me to scratch my head a little; however, don't worry about it-ignore it, and please don't work for anyone that would ask you to do such a thing. The bottom line is that they will fill in the contract with stipulations that are only to their advantage and not yours.
My advice? Never sign a contract until you have seen the school and your living quarters. Even then, insist on a 1 month trial period. Most importantly, as I've written several times in this forum, don't sell yourself cheap. It's naive to think that they can't pay you a decent wage and provide you with excellent benefits. If you believe in being altruistic, take your altruism to the very poor inland locations, accept the hardships of such an environment and do meaningful and worthwhile work for those who will appreciate your sacrifice. However, if you're like me and are not ready at this point in your life for such a sacrifice, recognize the value of your education and experience, and refuse to be exploited just because you've got a white face to go along with your TESOL Certificate.