8 x GPU under Windows is a lost cause.... but looking at your build, maybe relook at the PSU - suspect its cause of black screen....

I have a 7 x R7-370 built in June for ETH mining working like a charm.

Although the 370 is not really a fast card, its a very stable, super cool card and is not power hungry.

ATM, its mining ZEC at 58-60 H/s per card with Claymore v6 ZEC miner.

I spent so much time tweaking to get the 8th card to work but no good results.

I stopped experimenting after realising that those downtime should have been put to good use mining and getting profits. 7 cards not put in good mining use is really a waste...

Conclusion to share:

1. Beyond 7 GPU, only Linux can make this happen. Some dude in Ethereum forum has shared this in detailed.

2. Crazy multi GPU Linux rig, I think this dude went all the way to 18 (or more) x GPUs to mine ETH using a special USD$2000 PCI extension board usually meant for deep learning systems. Even Claymore had to release a special Linux version to support it for ETH mining.
See: https://bitcointalk.org/index.php?topic=1676763.msg16895926#msg16895926

3. For Windows 7,8.1,10 - max 7 cards with standard drivers is no issue esp with latest drivers. Older drivers, need the 6xGPU hack utility.

4. Windows Advanced Server with special drivers and Windows registry tweaking - 8 or more cards possible. Google "Deep Learning systems" that are Windows based.