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If you're only concerned about search engines there might be a slight advantage with the blue-widgets.com version, but if you intend to build a brand and maybe do some offline advertising, bluewidgets.com would likely work better. It's easier to remember, easier to say on the phone, and so on.
Search engines look at many, many other factors besides the domain name, so either version could rank just fine if you do enough other things right.
Whichever one you end up using, try to get control of both bluewidgets.com and blue-widgets.com, and maybe the .org and .net versions while you're at it. It's cheap insurance to protect your branding.