OK, thank you ! Great discovery !
I rephrase in a simpler way (useful for me, maybe for others) :
1. draw something ;
2. select it ;
3. define properties to "By block" ;
4. group it into a block (block definition) ;
5. change the block properties (block reference) in the property editor, it works !
5'. let the block properties (block reference) default options ("By layer"), then move the block to another layer, and the block take the properties of the new layer… Nice !
Is it possible to have "By block" by default, instead of selecting that option for each new line I draw ?