[fernando]

For as much as authors claim their knowledge and state their opinions as if they were facts, lack of susbtantial evidence often forms their strong adversary.
The black sword episode, as i suppose originally brought up, comes in HOMENS ESPADAS E TOMATES (page 164), by Rainer Daehnhardt
The conclusion that black swords were to prevent them from rust and also to prevent them from light refection, is his assumption. There is nothing written to state so; the name given in period inventories was ESPADAS PRETAS DE BORDO (board black swords). Assuming the rust prevention sounds logical, the double purpose of light reflection, which the author cites in first place, may be taken, nothing avoids, as just a romantic touch.
On the other hand, the 'colhona' swords 'convenientely' having their terminals sharpened to function as extra blades in a man to man fight, being also a quotation present in the same book, may only lack the term 'often' as nothing shows that they all had this intervention, but still has its veracity, as stated and surely verified in an example shown in the said work.
Maybe the down curved quillons issue has a more precise approach in this thread, but still interesting to notice how this phenomenom spread around, as shown (again) in the quoted book. The location and age of these examples attributed by the author is facultative.

#1 Sword of 1500's navigator, of Venetian origin.
#2 Sword breaker, also called left hand dagger of the reeds, Portuguese
influence in the Orient, XVIII-XIX centuries.
#3 Sword of Portuguese navigator, end XV century. Attributed to Pedro
Alvares Cabral (Brazil discoverer).
#4 Portuguese colonial sword, XVI century. with the magic number 1441
and the Passau wolf engraved in the blade.
#5 Portuguese colonial sword, with the round terminals sharpened and
perforated with the cross symbol.
#6 Portuguese colonial sword, with golden brass guard.
#7 Navigator sword second half XV century, used both in the Iberian
Peninsula as also by Italian peoples, then cultularly interconnected.

(All examples belonging in the R.D. collection)
.