I used the Hello World Custom Widget available in this tutorial, https://world.mendix.com/display/howto6/Creating+a+Basic+Hello+World+Custom+Widget as a foundation point and was able to modify the widget created from the AppStoreWidgetBoilerplate to change an attribute in my database. I was able to use the Mendix Client API in my widget to change and save an attribute in my database. I used Client API Mendix 5 but I think you should use Client API Mendix 6 https://apidocs.mendix.com/6/client/ because I think some things have been deprecated.
You should take a look at the slides of a webinar earlier this year, since you're using adobe brackets.
As far as the blank page goes; this means something's wrong with the widget you built. The best way to find out what is to open the browser's inspector (in Chrome it's F12).