My company is preparing electrical drawings for another firm.  We are creating backgrounds by attaching their building and piping drawings with XREF.  Their block for plug valves has a solid black triangle, which tends to hide our electrical linework.  I have tried to create a block in my drawing named the same as the plug valve block, without the solid triangle, and thought when the drawing was opened, it would redefine the block in the XREF, but with no luck.  Anybody know of a way to do this... redefine a block within an XREF, without redefining the block in the actual XREF .dwg file?  Thanks. 

Hello caddog7382, I have tried different ways of updating a block in the drawing with Xref, with a file appart ( like Wblock ) or in the Xref attached and it worked.
I have also tried to use the reference manager (Start, Program, Autodesk, AutoCAD Mechanical 2008,Reference Manager ). This is allowing you to do a refedit in the file with the Xref and this will update the attached Xref like you were openning the Xref instead.
 
I don't know where you are doing wrong ( with or with out the refedit command )!
On the other hand, maybe you should communicate with the other company because if you modify their Xref files and communicate yours, when they will open their modified files or yours WITH THEIR XREF? then the famous black triangle will appear again hiding your electrical linework.
 
Please, let us know if and HOW you solved your problem because Xref may really be an other planet.philippe JOSEPH2012-09-17 12:42:41

Thanks for your reply, Philippe,I have been able to redefine the block within the xref, but every time we receive updated background drawings, I am back to the big black triangle (and 36 and 48 inch piping have BIG black triangles), and need to go through and redefine the blocks all over.  I was hoping for a way to embed a block within my drawing, with the same name as the block in the XREF'd drawing, that would cause the XREF'd block to display the way I saved it in my drawing.