The blue highlight is essentially the text selection color as provided by the operating system (depending on your settings it may be something else).
There are some possibilities which may have happened in your form:
• the field got turned read-only (in that case, you won't be able to access the field without development tools (such as Acrobat). In this case, tabbing into the field won't work either.
• the field has some weird background color which cancels out the text selection
• the field has some code in it which prevent double-clicking to make a selection
In any case, things you can try instead of double-clicking:
• see if you can bring the text cursor (the I-beam) in the field and edit its contents.
• tab into the field and see if it selects the whole content or not, and whether you can edit
• triple-click (may help, maybe)
• click into the field and then press <Ctrl><A> (or <Cmd><A> on Mac) to select everything.
BTW, according to the description, this has nothing to do with the field highlight color (in fact, if you have it switched on, the color will disappear when you set the focus to a field, and by factory default, this is a light blue).
Hope this can help.