I worked around it by first plotting black lines and then the rest of the plot on top of it. This results in some lines being darker than in the legend, but it works for now.
plot(aa, bb,colour = :black, label = "High")
plot!(aa,cc,colour = :black, ls=:dashdot, label = "Low")
plot!(aa, bb,color=1,label = "Cl 1")
plot!(aa,cc,color=1,ls=:dashdot,label = false)
plot!(aa,dd,color=2,label = "Cl 2")
plot!(aa,ee,color=2,ls=:dashdot,label = false)
plot!(aa,ff,color=3,label = "Cl 3")
plot!(aa,gg,color=3,ls=:dashdot,label = false)
