Who is Sidney Blumenthal and why was Hillary Clinton contacting him via private email?

Sidney Blumenthal is the father of Max Blumenthal. Max is not a fan of Israel. It is peculiar how Sidney is such a neoliberal Hillary fan yet so defensive of Max. Clinton Adviser Sid Blumenthal's New Cause: His Son's Anti-Israel Book (2013):

A veteran confidant of Hillary Clinton has waded into a bitter argument over the explosive topic of Israel, defending his son’s intensely anti-Israel book from a liberal critic. 

Sidney Blumenthal, a former New Yorker writer turned Clinton adviser from the White House to the 2008 campaign, has been waging an online campaign against Nation columnist Eric Alterman for negatively reviewing his son Max Blumenthal’s book, Goliath. The book was described by Alterman, himself a frequent critic of Israel, as “awful” and something that “could have been published by the Hamas Book-of-the-Month Club (if it existed).”

Clinton Foundation officials refused to respond to questions of whether the elder Blumenthal is currently on a Clinton payroll.

Sidney contacted Hillary by private e-mail during the her destabilization of Libya because he had various business ideas, for himself, and wanted her feedback and insights. He stopped after the U.S. ambassador was killed at the consulate in Benghazi. Hillary thought Sidney was very insightful, and had been forwarding excerpts from his emails to her staff and friends for years. 

Sidney Blumenthal's Mosaic Mugshot by Kevin Vaughn, on Flickr

Sidney Blumenthal, a senior Hillary Clinton campaign adviser and former White House aide to Bill Clinton, was arrested in January 2008 for drunk driving in New Hampshire, one day before the state's presidential primary. 

Blumenthal, 59, was popped by Nashua cops after his rented Buick was pulled over for speeding. When Blumenthal, pictured in the below Nashua Police Department mug shot, showed signs of intoxication, a cop gave him a field sobriety test, which the Clinton operative failed. Blumenthal, who declined to take a Breathalyzer test, was booked into the Nashua lockup on an aggravated DWI charge.

via Kevin Vaughn, Mosaic Artist 20 January 2008

Sidney continued to figure prominently during Hillary Clinton's years in the Obama administration. He is quite a Washington D.C. socialite, appearing with prominent architectural firm CEO, Leo A. Daly III at the British Embassy in Washington, DC on 18 June 2009, to fete the Joffrey Ballet's new artistic director.

Joffrey_040 via UKinUSA on Flickr; Sidney on right

Sidney Blumenthal, Senior advisor of President Clinton by Lapo Pistelli, on Flickr in 1980 (middle)

Max, son of Sidney, appeared in ninth place on the 2013 Simon Wiesenthal Center list of anti-Semitic and anti-Israel slurs. 

In November 2014, Max was banned from ever entering the German parliament due to physically threatening and chasing senior German left-wing politician Gregor Gysi through the corridors of the Bundestag.

Sidney Blumenthal's notoriety, e.g. drunk driving, long history with the Clintons, son's poor behavior in Germany and Lebanon (newspaper Al-Akhbar fired him) could be cause for a less-than-public relationship with Mrs. Clinton.

EDIT

Apparently, Sidney Blumenthal is widely known and feared for his aggressive email campaigns about individuals of whom he disapproves, see Clinton Adviser Attacks Critic of Son’s Anti-Israel Book in Email Campaign. He has an infamous email list that he uses, à la ... well, I won't make any unflattering comparisons. According to the BuzzFeed article linked above,

Blumenthal’s list is a longstanding feature of the elite media landscape, a mix of liberal journalists, academics, and former Clinton administration officials, and has intermittently been the subject of controversy. 

“The thing about Sidney is that he’s absolutely obsessed with his enemies and his way of communicating and dealing with his obsession is that he sends out emails to this very elite list of liberal journalists mostly,” said one source who has been on Blumenthal’s list.