You're not doing anything wrong, that's the way that floating traps work, which I why I personally try to avoid them. They really do have a very specific role, and when you need it a floating trap is fabulous, but your case isn't it.
Instead I'd recommend that you find another trap and paint your 20 character address field normally. Then create a filter to isolate the addresses containing the "BLK" string, with an expression like this:
If you'd still like to go with the floating trap approach though, please post a more complete sample of the report and we'll give it a go.
In the Table window, you'll find an icon in the toolbar next to the calculator icon that has checkmarks and green and red dots. Click that to get the Filters dialog box. Click New, give the filter a name, like BLK and put the formula in the expression box.
Now OK that dialog, then OK the Filters dialog and save your model.