I see in your guidelines that you do not accept submissions of material previously published on-line including blogs of any kind. While I understand why you would require this, wanting your magazine to present new material for the reading public, I believe there may be a few things you didn't consider when crafting your guidelines.
I (and certainly if I am doing this, many others are too) use the blog format as a way to organize work that is more or less finished (revisions of course are always necessary). Before the blog, I wrote a lot and finished little, knowing that most of what I was writing did not fall into easy categories and was therefore "un-submittable". And because there was no easy genre to assign the writing to, I found it very difficult to be organized with my writing, though I tried many things, file folders, binders, stacks, boxes, etc. Inevitably, work would get buried, lost, forgotten.
Now, to make new work and not put it on the blog, particularly "micro-fiction"which is well suited for the blog format, becomes very difficult since I have no good way to keep track of work I don't post to my blog and because if I didn't publish the work on my blog, hoping to get it published in literary magazines, than I would never have any work to put on the blog since as we know the submission process is long, taking months to hear back about a particular piece and more likely, years of submitting until a piece gets picked up and then I would be back to where I was before the blog but with more complications being I would have to try to figure which pieces to put on the blog and which not to and how to keep track of all that. So, posting finished pieces to my blog makes it much easier for me and leaves me more time and energy to make new work which is why we all get into the game in the first place.
In my case (and certainly, if it is true for me, it is true for many others as well) the 25 to 125 blog hits I get a day, when excluding myself, spy-bots, relatives, and a few close friends, leaves anywhere from 0 to 20 others who might visit my blog on any given day and of those, how many actually take the time to read it? Plus, the work I would like to submit to your contest is buried so deep in the archive that even if anyone did read it, they have long forgotten. So, you need not worry that publishing work from my blog in your literary magazine would compromise the freshness of your content.
True there are many things about your current guidelines that have not occurred to me to consider. But these are merely the things about your current guidelines that have occurred to me that I hope you might consider.