What is "too dark"? Like blackish blue? It really depends on how you set it. A bezel for instance will probably make it look darker than a diamond halo, KWIM?
It already looks quite dark and I wonder how much of that blue would actually show if you''d ask for additional photos. You can get the same (if not nicer) shade of deep, velvety blue by getting just about any quality grade of black opal. And those that show only blue on black background are really inexpensive, so it''s something I''d suggest you consider. Stone #330 on this page:
Yes for me it would be too dark. I think alot of light was shone onto the stone during the photo. So the center is lighter than it would be normally. Ask for more photos and see what you think!
I personally prefer lighter stones but it could be a velvety blue and you might like that. HTH