A blue slip or blue-slipping refers to two different legislative procedures in the United States Congress.
In the House, it refers to the rejection slip given to Senate tax and spending bills which have not originated in the House in the first place, per the House's interpretation of the Origination clause.
The Senate can simply circumvent this requirement by substituting the text of any bill previously passed by the house with the text of a revenue bill, as was done with H.R. 1424.
When, in the opinion of the House of Representatives, a Senate-introduced bill that raises revenue or appropriates money is passed by the Senate and sent to the House for its consideration, the House places a blue slip on the legislation which notes the House's constitutional prerogrative and immediately returns it to the Senate without taking further action. This blue-slipping procedure, done by an order of the House, is routinely completed to enforce its interpretation that the House is the sole body to introduce revenue or appropriations legislation. The failure of the House to consider the legislation means it cannot become a law. This tactic has historically proven of great use to the House and, as a practical matter, the Senate does not introduce tax or revenue measures to avoid a blue slip.
Professor Brandon Denning has suggested that "the executive branch take a more active role in identifying and publicizing what it perceives to be abuses of the procedure during the confirmation process.”