If you connect to just about any site running TBdev derivative code you'll find that 6881 (and most of the ports below 10000) are blocked by default. Most of the coders on those sites aren't even aware that the default config blocks these ports. I can't speak to other codebases but for TBdev the "standard" port blacklist includes 411, 413, 1214, 4662, 6881-6889, 6346, 6347 and 6699. Most sites recommend using ports over 10000.