Is citing a blog post allowed in a research project? I'm currently writing an essay on the possible benefits of video games in education for my Extended Project Qualification, and for part of it I want to cite a blog post from Gamasutra written by a developer who makes educational games. My supervisor has previously said to me that I should try to stick to citing research papers or books, as these are generally reputable, and I agree with this. However I cannot find anything in a journal that fits the point I want to make, as I wanted to write about a developer perspective on these kinds of games. Would citing such a post have an impact on the reputability of what I am writing, and therefore impact my grade?
This is fine. Citing someone's opinion is different from reusing someone's results as long as the former is clearly separated from the latter. Find a way to distinguish both kinds of citations in your essay. The former is the data on which you operate, after all. 

If your advisor insists of such a separation, consider using footnotes, endnotes, or an extra list (in addition to the bibliography).