Both sentences are grammatically sound, however, if I were to use Sentence No. 1, I would put an “a” before “further.” This emphasizes that the lines, which are already blurred, are even more so by whatever “this” is. In terms of their usage, as Chris noted earlier, a Google Ngram Viewer search shows it being approx. 2 to 1 for the latter, while a COCA search shows it being slightly more. Thus, I would say sentence No. 2 is the more natural as more native speaker (and writers) use it.