Form On Top of Form

In my program I have a child form with a textbox that is displayed when a MenuItem is clicked. I want it to display on top of the parent form, but what is happening is that you can see the parent form right through it, making the textbox impossible to read. Both forms have the Opacity set to 100%. I fiddled with setting the Topmost property but that didn't do it.
How can I display a child form on top of a parent form without taking down the parent?

VB Code:

Dim f As New SomeForm
Me.Enabled = False
f.ShowDialog(Me)

You may also want to check that you haven't fiddled with the TransparencyKey property of the Forms.