The website judicialnominations.org explains the process:
One way in which senatorial courtesy has manifested itself is something called the “blue slip.” This is a device used by the Senate Judiciary Committee to communicate with the home-state Senators about a nomination to the U.S. courts of appeal or district courts, or to be a U.S. marshal or U.S. attorney. When a nominee is referred to the committee, the committee sends a letter (typically on light blue paper) asking the two home-state Senators to take a position on the nomination. The Senators check off the appropriate box on the sheet—either approve or disapprove—and return the paper to the Judiciary Committee.
The blue slip process is used only by the Senate Judiciary Committee —no other Senate committee uses it for other kinds of nominations. The practice of using blue slips dates back to at least 1917. Since mid-2001, the status of blue slips for each judge nominated have been publicly available on the Web.
It is a matter of some debate how important blue slips are in the confirmation process. The blue slip practice is not a formal part of the Judiciary Committee’s rules, and the determination of just how much weight to give to a Senator’s opposition to a nomination is left largely up to the chair of the committee. Among other issues, the chair will decide whether to honor the objections, voiced through blue-slips, from all home-state senators or just those who belong to the same party as the president.
Unfortunately, the process has been occasionally abused. The Judicial Nominations website explains:
Much also has been written that is critical of the blue-slip system. George Washington University law professor Jonathan Turley described the system this way: Blue-slipping is a little known process by which senators can block federal judge nominees from their state. This means that judges who may rule in your case often are selected to meet senatorial, not professional, demands. By simply not returning blue slips sent by the Senate Judiciary Committee, a senator can block a nominee for the most nefarious or arbitrary reasons, including a personal grudge, a bargaining tool with the White House or failure of the nominee to be sufficiently fawning in the senator’s presence.
This courtesy has been misused by both sides–it was not meant as a negotiating tool–it was meant to be a courtesy.
The article at Power Line details some changes that Senator Grassley is making in order to expedite the confirmation of President Trump’s judicial nominees.
Power Line reports:
…Senate Judiciary Committee Chairman Chuck Grassley announced that he would not let Franken’s withheld blue slip block the nomination of Minnesota Supreme Court Justice David Stras to the Eighth Circuit (or Senator Kennedy’s block Kyle Duncan to the Fifth Circuit).
Senator Grassley took to the floor of the Senate to explain his disposition of “the blue slip courtesy” and his decision to schedule a committee hearing on the nominations of Stras and Duncan (text of statement here, video below). The Hill reported on Senator Grassley’s statement here.
Washington needs to stop playing games and get its work done. All Congressmen (and Congresswomen) should be paid according to what they actually accomplish. That might actually change how things are done in Congress.