It bore an engraved escutcheon, a herald's wording of which may serve for a motto and brief description of our now concluded legend; so sombre is it, and relieved only by one ever-glowing point of light gloomier than the shadow: "ON A FIELD, SABLE, THE LETTER A, GULES"
I looked them up (the words in bold) in online dictionary websites but I could not pick up the most correct meaning they have to fit into the above sentence.
Could you explain the meaning in the sentence in italic ? Thank you.