Durmstrang Ship 4768 was released in 2005. To the best of my knowledge, the last previous set that contained black falcon shields was Black Falcon's Fortress 10039 in 2002 re-released as part of the short lived legends line. It contained one of each black falcon shield variety, blue bordered and yellow bordered. Before that, Guarded Inn 10000 was re-released in 2001 and contained only the yellow bordered variety.
Here is where we have to make assumptions. I bet they printed the same amount of shield varieties for whatever reason (maybe simplicity, or banking on the legends line being more successful and therefore more re-releases), leaving a surplus of blue bordered ones. Not wanting to waste a part, the blue bordered variety ended up in set 4768 on the advice of a savvy LEGO accountant.
Personally, I find the reuse of that shield a great way to snag more sigils of a well loved faction from the past and consider it more of a blessing than laziness. Just my take though.