Question: Is there a downside about taking too many short-term contract positions? I'm still early in my career (I'm 25) but I've only been in a perm full-time job once for 1.5 years. My other 2 jobs have been been 6 month contracts. My current job is a 6-month contract-to-hire position which has been ok but I'd like to move elsewhere when the contract finishes. Are all these short-term jobs seen as a black mark? 

I've read that it's recommended to stay at a permanent full-time job for a year but don't contractor jobs have different guidelines? Isn't it common for contractors to jump around jobs like this?

Answer: 
I wouldn't view you as a job hopper and disqualify you on that basis, but, to be honest, I would have difficulty considering you qualified for a senior position either. This is because you haven't had to live with the results of what you created. Further, many people I have interviewed and worked with as contractors with this type of experience never develop the depth to become senior developers, they are repeating the same level of experience over and over. Now if each contract takes you a little further in depth on something and you are clearly developing significant expertise in an area, then it would not be a deal breaker particularly if you are a specialist of some kind. But the interview would be quite pointed towards how you have grown and learned from your mistakes.
Most hiring managers wouldn't consider that to be a black mark. Deliberately changing jobs every year or couple years is viewed very negatively by a lot of hiring managers though. (A survey of recruiters and hiring managers found that 39% of them listed "job-hopping" as the biggest obstacle for job seekers. <http://www.askamanager.org/2012/10/job-hopping-is-killing-your-career.html>)

Because of that, I think it's worth listing the temporary jobs as such on your resume, for example by putting "(6-Month Contract)" after the dates of employment. This can prevent someone who's worked a lot of contract jobs from being viewed as someone who gets bored after a few months and moves on.