I haven't watched this, but from the context, I think you may have misheard. 'I filled in the blanks' would make more sense and it means that she made assumptions about what he was like, rather than really getting to know him. 'The blanks' refers to the things/information she doesn't know/the 'missing information' 'to fill (them) in' she needed to get to know him better/get more information/answers. Because she didn't take the time to get to know him/get more information she made assumptions about his character (filled in the blanks herself based on what she already knew about him) and as a result the marriage was a disaster (she married someone she barely knew). I hope that makes sense.... :)