Render using GGX BRDF. Not sure if I botched the implementation but was trying to get nice metallic colors for the two spheres.

Z-Buffer render of the scene for reference.

Render using Phong BRDF. Looks more plastic and old school than the Blinn for sure.

Final render using Blinn BRDF, took 0:00:00 seconds, using 8 threads in a Ryzen 7 7700 @ 5.2 Ghz.

Interesting bug: was adding negative colors by not discarding rays that hit from the back.