I assume by EDMX you mean Database First or Model First rather than Code First (POCO). You can still do code first with POCO's with an existing database if that's the reason for using EDMX.
However if you're absolutely stuck with an EDMX you can use POCO data transfer objects as a wrapper around the EDMX to isolate it from your BLL and top UI. This question about the same subjectThis question about the same subject on Stack Overflow discusses this alternative.