Dear Mr. B:
You wrote:
>Question: What do you call the noun in a prepositional phrase? If a sentence reads, “The XYZ blog has a great blog post about John Smith,” what is ‘John Smith’?
>My understanding is that ‘XYZ blog’ is the subject, and ‘blog post’ is the direct object. But ‘John Smith’ would not be the indirect object, right? If not, is there a word for such a noun in a prepositional phrase?

The noun in the prepositional phrase is the object of the preposition. In the example you gave, “John Smith” would be the object of the preposition “about.”

Prepositions got their name because they normally are in a position before a noun (a “pre” position).