Blackstone Group LP has an enticing pitch for the ultra wealthy: Invest in hedge funds and avoid paying taxes forever. The private-equity firm’s Lombard International is leading the charge to promote exclusive insurance vehicles that can contain hedge fund investments. Lombard is urging hedge funds to sign on and working with private banks such as JPMorgan Chase & Co. and Bank of New York Mellon Corp. to market the investments to their rich customers. The pitch is working. Clients have put about $3 billion into the products in the past year, with Lombard attracting most of that business.