By Anirban Sen and Rajesh Kumar Singh (Reuters) – Spirit Airlines is seeking to convince JetBlue Airways to appeal a decision by a federal judge to block the tie-up between the sixth and seventh largest U.S. airlines, according to people familiar with the discussions. Spirit’s shares have lost more than 60% of their value since U.S. District Judge William Young ruled on Tuesday that the company’s $3.8 billion sale to JetBlue should not proceed, siding with antitrust regulators who argued the deal would lead to higher fares for passengers. The value of Spirit’s bonds have also plunged in value,…