The identity of Mr.Blank, the influential Irishman of chapter 25, is a
mystery.  Miriam Shillingsburg published an article on him in the Mark
Twain Journal in 1993.  Unfortunately for me, I don't have access to
this article save for the first page.  It is available through Proquest
if your institution has a subscription.  I don't have an institution. 
 I also regret that I cannot affort a subscription to the Mark Twain
Journal, or any other scholarly journal for that matter.  It is a most
worthwhile endeavor and I do  own a couple of issues pertaining to the
Innocents Abroad.  Anyway, Barbara Schmidt reports that Ms.
Shillingsburg theorizes that Mr. Blank is one of three possible
candidates.  None of them are Mt. Charles Casey of Pollerton Castle,
Carlow, Ireland.  Mention of Mr. Casey can be found in the Google
teaser for this publication so Ms. Shillingsburg was obviously aware of
him.

It was with Mr. Casey that Sam Clemens had correspondence, May 15,
1876.  “Of course I don't know what you hold, but I “call” you anyway! 
 This being translated means:  Tell me about this thing.  Really &
truly, now, is there a Twain Club?”  This sounds just like the fellow
Twain is describing in Chapter 25 of Following the Equator and
ascribing to Mr. Blank.

However, looking into the scant details of Mr. Casey it seems unlikely
that he would have traveled to Bendigo, Australia.  Also, it seems Mr.
Casey was not a bachelor but he was a most curious character.  Aside
from being a Mark Twain fan, he was quite interested in the Great
Pyramid and published a book about its “use and meaning”.  “Philitis, a
condensed account of the use and meaning of the Great Pyramid”. First
published in 1872, it went through several editions until the fifth in
1880.  Casey had also written about Darwinism.  I don't know if he was
thinking about Charles Darwin or Herbert Spencer.

It's possible Mr. Casey became interested in the Great Pyramid from
reading  “The Innocents Abroad”.  There is an entry in the “Papers of
Ulysses S. Grant” dated 1873, July 4, a request from Charles Casey,
Carlow, Ireland to USG:  “I very much desire some official recognition
of my existence as a U.S. citizen during my exile in the Island of Pigs
Politics – Punch – & potatoes, and would be pleased at some unpaid
appointment as that conceded to “Mark Twains' fellow passenger on the
Quaker City – but not quite so wide in scope...”  He remarks on having
two sons, ages 8 and 6 years.

It's possible the Mr. Blank was indeed the author of the reported death
of Mark Twain in Melbourne and I suppose it is possible he did sail
with Henry Bascomb, around the world.  But, I suspect Mark Twain
conflated the Mark Twain Club of Carrington/Pollerton castle with Mr.
Blanks' antics because he wanted to tell the story of the club and Mr.
Blank provided a convenient Irishman for the job, nevermind that Casey
was actually an American expatriot.