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Sorry but I many times changed between Android 6 and 7 with SP Flash Tool for test purposes but never had a black screen. Something must be wrong with your PC setup, drivers or USB connection, or even with the hardware of your phone.
You can't downgrade Android version with any kind of recovery method.
You still can try to format the whole flash chip under the Format tab in SPFT, then flash the android 6 ROM but if you do so, then you have to restore your IMEI numbers with SN Writer Tool.
also I must repeat again, the latest Android 7 version has absolutely no stability issues. If you have such experiences, that's a specific issue on your side.
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