Hmm... I'm going to try to be gentle here, but honestly, absolutely nothing you've said here is correct. I'll take it bit by bit, and see if we can correct some misunderstandings.
Since the Ninja evolved from the Samurai
Hmm, no. The first thing to realise is that what you think is a "ninja" is largely a fiction... historically, a "ninja"/"shinobi no mono"/"any similar or related term" was someone who performed, or assisted in performing, particular tasks, associated with espionage, information gathering, and infiltration. Many times, these persons were samurai, simply filling a particular role in their duties, which might include "ninjutsu" skills such as tracking, scouting, reconnaissance, and so on. While there were some areas that were particularly reknowned for such skills (Iga and Koga), the warriors that came from there were commonly referred to as "Iga no mono" (person from Iga), or "Iga no Bushi" (warrior from Iga), rather than anything specific about them being "ninja".
The samurai were the warrior class of Japan from the 12th Century until the mid-late 19th (roughly), but exactly what a "samurai" was depended greatly on the time period you're looking at... the samurai of the Kamakura period weren't the same as the samurai of the Sengoku Jidai, or of the Edo Jidai. Throughout, however, they were the ones in charge of military matters, in one way or another, and, as they were at the upper end of the feudal caste system of Japan, there was nothing that really "evolved" from them... it'd be a step down to go anywhere from samurai status, really.
I though that the Ninja sword arts were much the same as the Samurai sword arts.
Well, having first established that there's no real difference between ninja and samurai (historically), I think it's pretty clear that, well, no. And, realistically, there is no such thing as "ninja sword arts"... there are particular sword systems that have a unique, or individual approach... but the distinction is the same as between any two "samurai" sword arts.
However, the swords are a bit different.
No, they're not. Individual schools might have a preference for particular sword designs, furniture, configurations, and so on, to the point of having a speciality sword in some cases, but in the end, that's more a particular schools preference, and that's it.
The Samurai sword has a round guard and a curved blade
That's a very generalized statement, and not a really correct one. For one thing, there are any number of tsuba (sword guard) designs, some are round, some are square, some have particular identifying characteristics, some are oddly shaped, some have holes, some are more solid, and so on. And some swords didn't have tsuba at all.
and the Ninja sword has a square guard and a straight blade,
No, that's a product of essentially Hollywood myth. That sword never actually existed.
so I would think there must be some difference in how the swords are used.
There are differences between individual sword arts, but that's all. If you're not talking about specific schools, then you really can't talk about differences in usage at all. Hyoho Niten Ichi Ryu has methodologies that are different to Tenshinsho Den Katori Shinto Ryu, which are different to Yakumaru Jigen Ryu, which is different to Kashima Shinden Jikishinkage Ryu, which is different to Yagyu Shinkage Ryu, which is different to Maniwa-Nen Ryu, which is different to Muso Jikiden Eishin Ryu, which is different to Tenshinsho Jigen Ryu, which is different to Mugai Ryu, which is different to Araki Ryu, which is different to... well, all the others. And none of these are "ninja sword" schools, although some contain "ninjutsu" teachings as part of their methods. Within the modern Ninjutsu schools, the Togakure Ryu is said to be a "ninja" system, and features a sword methodology, but it's differences aren't any more different to the other sword system taught, Kukishinden Ryu, or Kage Ryu, or anything else. It just has it's own unique approach, same as any other system around.