Mirror BlockReference on UCS

I having problems mirroring blockreferences when the active coordinate system is not the WCS.  We are mirroring through the base point of the blockreference.  On the WCS I am doing this:

Pt = Blk.Position.Add(New Vector3d(0, 50, 0))

lin = New Line3d(Blk.Position, Pt)

Blk.TransformBy(Matrix3d.Mirroring(lin))

 For the WCS it works very well, however when the current coordinate system is a UCS then it does not work.  I have tried this:

mtrx = Ed.CurrentUserCoordinateSystem

Dim vt As New Vector3d(0, 50, 0)

'vt.TransformBy(mtrx)

Pt = Blk.Position.Add(vt)

lin = New Line3d(InsPt, Pt)

lin.TransformBy(mtrx)

Blk.TransformBy(Matrix3d.Mirroring(lin))

 I think my problem is the end point of the line, pt.  I tried to transform the vt but that does not work either.  Can someone please help me here.  Thanks in advance.