I am sorry, but it’s a design flaw to marry a block definition to any layer–because it doesn’t have to be on any.
After dealing with this, I now create ALL of my definitions on the “default” layer. From there, I put the instances on which layer I want. It’s a poor compromise, but it’s better than searching for definitions.
[This is one of the only 3 things I dislike about Rhino, the others being the texture replacements didn’t work in blocks, and the block manager is quite paltry for counting objects.]