Agree with OB and CH on boiling water not being hot enough to have any effect on steel heat treat.

Two additional thoughts. The dark color in the engraving is likely "dirt." So, soap/solvent cleaning with a mechanical scrub may be effective. Clean engraving on a "coin" finished receiver doesn't show up well. New guns with fine line engraving and coin/grey finish are sometimes "inked" to make the engraving show. Take care how thoroughly you clean the engraving. I doubt that there is any damaging corrosion going on in the bottoms of the engraving cuts.