The Obituary for Robert Ball Hughes has eluded me for several years. I finally found it through Google News Archives on p. 2 of the Boston Daily Evening Transcript for Friday, March 6, 1868.
It turns out I had most of the text already on the Biography page on RobertBallHughes.com. It was quoted in The New England Historical & Genealogical Register For the Year 1868, Vol. XXII, p. 185, after Ball Hughe's death. Part of the first sentence and one complete sentence starting with “For some time past…” were omitted.
I could not find this article directly with Google Search. The digitizing of newspapers is not always accurate due to the condition of the paper and the limitations of character recognition software.
After finding the daily issues of the newspaper for March 1868, it was a matter of reading an entire paper, starting with March 5th issue, to find the article about Ball Hughes. This can be a tedious, especially if it turns out that there is no information to be found.
Since the obituary was not in the March 5th issue, I hoped that it might be in the March 6th issue, and it was. It helps to have a range of dates to narrow down the search.
I also found another obituary from The (London) Art-Journal that was based on the New York Tribune obituary that I have not seen yet.
Read more at the Obituary for Robert Ball Hughes on RobertBallHughes.com.