For polarization-division-multiplexing coherent optical orthogonal frequency-division-multiplexing (PDM-CO-OFDM) systems, two blind phase estimation algorithms, including the decision-directed (DD) scheme and the DD plus decision-feedback (DF) scheme, are proposed. The proposed algorithms are verified in a 112 Gb/s PDM-CO-OFDM system, and compared with the traditional pilot-aided (PA) algorithm. The simulation results show that the DD algorithm can achieve better performance, but is less tolerant to laser linewidth, the DD+DF algorithm can greatly improve the performance and the laser-linewidth tolerance of the DD algorithm ,and the phase recovery result is much better than PA algorithm using five pilots.