Have you actually debugged your code by place a break point in the BeginDoublClick event handler? If you did, you would know whether your code is actually work or not. Attached is the picutre of running my code, where the selected entity's ObjectID is shown in command line. What I did is
1. Load the code I showed in previous reply;
2. draw a polyline
3. issue command "MyDbClick"
4. double-click the polyline
However, handling double-click in your code, does not stop AutoCAD from doing its own double-click action as configured (in your case, double-clicking an block's attribute opens attribute editing dialog box; in my case, double-clicking a polyline starts _pedit command), because your code just register a new doubl-clicking event handler, which does not affect any existing double-clicking event handlers.
If you want to change AutoCAD's doule-clicking behaviour and acts only according to your double-click behaviour, you need to change the DoubleClick action, defined in CUI. You can also use code to make this kind of changed. Simply handling BeginDoubleClick event will only add one more behaviour on top of existing behaviours, as we have already seen.
Or if you do not want AutoCAD's default DoubleClick action, you can simply go to Options->User Preferences and uncheck "Double Click Editing" in the risk of making many CAD users very angry unless your double-click actions satisfy your user better.