So you get a blu screen when 8GB is installed?
And, changing to 4GB, it boots and works fine?
That's not too difficult. One or both of the RAM sticks for 8GB are faulty, or are not quite compatible with your MBPro.
Or - could be the logic board... Why did you replace that? Was it graphics or video problems?
If you have either a 15 or 17-inch model, that's a pretty common issue, and a replacement logic board may not necessarily fix any graphics issues on a 2011 model.
If the RAM is the issue, then consider that any 2011 model can be upgraded to 16GB (2 x 8GB), but, I'm not sure about spending that much on a 7 or 8 year-old laptop.