We consider the use of free-space optical communication for fast moving platforms such as high-speed trains where the sampling clock offset is randomly changing and, in addition, the receiver does not have any information on the instantaneous channel fading coefficient. By employing multiple samplers at the receiver, we propose a class of sequence detection methods for the case of OOK signaling without using any training sequence. First, we study maximum likelihood based detection, which has a relatively high computational complexity. Second, by employing generalized likelihood ratio test, we propose a more practical blind sequence detection method of reduced complexity. To further reduce the computational complexity, third, we propose a novel scheme that uses two wavelengths at the transmitter and differential blind detection at the receiver. Fourth, to benefit from diversity gain with this differential scheme, we consider the use of sufficiently different wavelengths, where we propose an efficient blind detection method. The pros and cons of each proposed detection method are contrasted through numerical results and their processing load are compared as well.