Traditionally the black flag isnít no surrender. Itís kill everyone no prisoners, no mercy. Flying the black flag is supposed to be one of the worst things anyone can do. So it would be diplomatically difficult. Plus it defeats the whole purpose of a coat of arms or any badge, itís easy to miss in bad weather itís indistinct. Iíll crack out my old heraldry books and draw something up once the family history has been decided. After all history often has an impact on the coat of arms. Same for the motto, lets wait until the history has been fully fleshed out before we decide upon it.
Addition by edit: Also might I request that you make Guyard Rensyn older. The reason I say that is 17 makes all other family options very limited. One you can only really have one younger sibling who is old enough to do anything of interest and aside from the lord and the lord's siblings no one else can be much older. All the lord's brothers will be younger than the lord and so their children likely will be as well. I might be wrong but it seems like you are really stunting the realistically playable family tree by being 17.