JetBlue Airways Corp.’s stock was the rare spot of green on stock screens Tuesday, getting a boost from a fresh investment by billionaire activist investor Carl Icahn. JetBlue JBLU, +15.90% shares were on track for their highest close since Aug. 2, when they closed at $6.90, and their largest one-day percentage increase since Dec. 7, when they rose more than 15%. Icahn disclosed a 9.9% stake in JetBlue in a filing late Monday, saying that the airline’s stock was “undervalued and represented an attractive in ...