Final answer:
The Golgi stain specifically colors the neuronal structures black, most notably the Golgi bodies or Golgi vesicles within nerve cells, rather than any of the individual options presented in the question (nuclei, cytoplasm, mitochondria, endoplasmic reticulum).
Step-by-step explanation:
The neuronal staining technique known as the Golgi stain fills the neuronal structure with a black or silver color, primarily highlighting neuron bodies and dendrites. The cells that appear black are the ones with a network of vesicles that we now refer to as Golgi bodies or Golgi vesicles. This staining technique was developed by Camillo Golgi, an early neurobiologist, who used it to detect these vesicles in nerve cells. Golgi vesicles are a part of the endomembrane system, which includes various organelles such as the plasma membrane, ribosomes, rough and smooth endoplasmic reticulum, and actin filaments. While not included in the options given in the student's question, it is important to understand that the Golgi stain specifically stains neuronal structures, rather than individual cell components like nuclei or mitochondria.