1. make the difference(s) between things less clearly defined
blur the lines between fact and fiction
When things are blurry, they are out of focus and are difficult to make sense of. When you 'blur the lines' between something, you make it difficult for people to see or be aware of where the boundaries are. In this case, 'the lines' are the differences that separate two things, like ideas, situations, or beliefs. You can 'blur the lines' between right and wrong; or between 'true and false' or 'fact and fiction'. What this means is that the clear distinctions between these opposites are no longer clear.