Bler is the Block Error Rate.
When we are using Harq ,
Downlink Bler = ( Number of Nacks ) / (Number of Acks + Number of Nacks)
Downlink Bler can be computed dynamically and the system can try to aim for a "target bler". Target Blers will be different for PDSCH and PDCCH.
Generally the target bler is calculated w.r.t the first transmission (=aka a new transmission). If the target bler is 10% then the system aims for a Bler <= 10%
A residual bler is calculated w.r.t a retransmission. It can also be used as a performance target.