I wouldn't accuse Honda of using "such a poor steel" because they rust quickly in air/water, that's what most steel does, particularly certain steels with higher physical properties and especially if you've just rubbed them with abrasive. In the environment inside the engine it won't, in fact it's often bad practice to electroplate things used inside engines (potential embrittlement effects), and high spec fasteners like con-rod bolts or main bearing bolts are usually phosphated or similar. You may find the blackish colour of the plates is a surface treatment like phosphate etc.
Oil specs