again some of the same comments apply to double drop d,dadgbd,it has similarities with dadgad,in that you have three d strings,so of course dropping the octaves apply the same,as regards your 4th string to either your sixth or up to your first.
with double drop d,you have four strings the same ,so even more chords that are common to standard are available.
as regards playing blues in double drop d,the key of A major is great,and EASILY produces sounds reminiscemnt of Memphis Minnie.the article from Martin Simpson is a great one and thanks again.Dick Miles