I know they are confused and have been indoctrinated by the APA to call lavender "self blue" and our Alliance has nothing to do with that recognition process.
I can address the questions about lavender Ameraucanas though.
By definition lavender dilutes black, but in reality it dilutes much more, like red feathers, dilutes shank color and eye color.
Since E based shanks should be darker than ER and assuming lavender would dilute them equally, a lavender with based on E would have darker shanks than on based on ER.
I believe what you are seeing as black shanks on lavenders are really slightly diluted from black, the darkest shade of gray (?), to something a little lighter.
Some do still breed blues to blacks, but the current opinion is that the best blues are based on birchen (E
R) while the best blacks, lavenders and whites should be based on Extended black (E). E produces the best solid colored varieties. Blue is really Laced Blue, so it isn't a solid color and some assume E
R is a better base color/pattern to build it on.
Further research by Dr Okimoto (quote from Classroom @ The Coop: Blue & E/ or ER/, posted May 02, 2006) ......
Some of us speculated that Andalusian blue used ER instead of E because ER would be more amenable to secondary pattern genes like Pg. I recently tested some Andalusian Blues from McMurray. I had a mixed batch of chicks so I could only tell the gray ones as blues the blacks could have been something else. There were three gray chicks that produced gray chick feathers. Two were homozygous ER ER and the third was heterozygous E ER. My guess is that E probably causes the dark birds and that the show quality birds are probably ER.