It is, if it clearly IS a border, not a vignette. A very gradual fade-out to a black or dark border is the same as a strong vignette, a sharp transition to a dark or black edge is a border, and somewhere in between is a gray area :-P
Historically (except very long ago) we've allowed most things in borders except other images/art and text, but it still must qualify as a border. So whatever you do, if it is quite recognizable as a border, and does not contain another image or artwork, nor text, you're good to go.
Now, that all said, the voters don't often take kindly to "creative borders," so keep that in mind :-)
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