So it's a blake 256 8 round
No. It's Blake256 14-rounds with (obviously) some modified headers.
The closest miner for this algo for AMD is here: https://github.com/nicehash/cgminer-blake256r14/releases/tag/3.7.2-nicehash and the closest miner for NVIDIA is here: https://github.com/tpruvot/ccminer/releases/tag/1.7.0-tpruvot ... However, you can test these miners only to get a raw speed estimation of your rigs, because the final version of mining software will obviously be slight different and none of the existing mining software will be compatible with Decred.