Convert a string to a DependencyObject

Alright, i've googled this until my computer was blue in the face and have gotten nowhere.

I want to assign a behaviour to some textboxes through a for loop. I added a messagebox to make sure the BlockName.Name was correct...it is. The TextBoxes are already created and named bl1, bl2, etc. I only need to assign the behaviour to 4 on this tab, but others have 20+. With the following i get "cannot convert from string to System.Windows.DependencyObject.

public MainPage()
{
InitializeComponent();
for (int a = 1; a < 4; ++a)
{
TextBox BlockName = new TextBox();
BlockName.Name = "bl" + a;
MessageBox.Show(BlockName.Name.ToString());
Interaction.GetBehaviors(BlockName.Name).Add(new Watermark());
}
}

If i change the last line to:
Interaction.GetBehaviors(BlockName).Add(new Watermark());
I do not get the effect. If i assign each box individually it works perfectly. What am I doing wrong???