What do you mean by blank pixels? Cycles is a MC path tracer. This means that it will always trace some paths that don't find high contributions to the final im

What do you mean by blank pixels? Cycles is a MC path tracer. This means that it will always trace some paths that don't find high contributions to the final image. When this happens for the first couple of iterations when rendering progressively, these pixels stay darker than the rest. This is just how the algorithm works. You could try to apply a denoiser, but then you're not using the computational power to shoot more rays and you're replacing noise with bias.