BLD is the Robinson-Foulds distance weighted by branch length. Instead of summing
the differences in partitions between the two trees, the metric takes the square root
of the squared difference in branch lengths. Parallelizable.
References
Kuhner, M. K. and Felsenstein, J. (1994) Simulation comparison of phylogeny
algorithms under equal and unequal evolutionary rates. Molecular Biology and
Evolution, 11, 459-468.