As the technology for 100G data rate matures and transponder deployments take-off, research efforts are being directed towards determining the appropriate channel formats for future transport data rates. Realizing 400G will likely be attained via multiple 100G/200G carriers and exploiting a DWDM flexible grid to maximize spectral efficiency gains. This paper overviews the channel formats under discussion to support 400G. Based on their main characteristics, such as expected transmission reach, spectral occupation and routing restrictions, the blocking performance of DWDM networks under dynamic traffic is evaluated. The analysis considers the performance impact of the number of transponders deployed at the network nodes and the properties of the network topology used, which ranges from a single link to metropolitan, regional and long-haul networks.