According to the experts (Burke 2013, Forsyth 2004), Arion ater and Arion rufus can only be separated by dissection and comparing genitalia. The black morph of Arion rufus is not uncommon.
Burke, T. E. (2013). Land snails and slugs of the Pacific Northwest. Oregon State University Press.
Forsyth, R. G. (2004). Land Snails of British Columbia. Royal BC Museum.