In this letter, the non-coherent grant-free non-orthogonal multiple access transmission in the block fading channel is studied, and a transmission scheme using pilot and channel block-index modulation (IM) is proposed. In the proposed scheme, the transmit frame is divided into multiple channel blocks and each user is allocated multiple pilot sequences. In data transmission, only part of active channel blocks are active and further only one pilot sequence is active in each active channel block. The indices of both active channel blocks and pilot sequences carry data information by utilizing the principle of IM. At receiver, the non-Bayesian maximum-likelihood detection with adaptive threshold is presented to perform the joint user activity and data detection. Finally, simulation results are given to demonstrate the performance of the proposed scheme.