The little ribbon in the back must be covered. See picture. Remove the extra foam tape at the bottom. Also, the darkening is usually caused by a bad IC chip. You will need a factory one, or you can buy a new one, which means you want to buy quality. Not something cheap because cheap IC chips will do that. Check the soldering job twice. That could be the problem. Clean everything, including the connectors.
Read [http://blog.etechparts.com/2013/07/30/troubleshooting-the-repair-ipad-mini/|THIS] blog.
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