The Winchester's were so stupid.
Everybody knew that Harry Potter made Voldemort immune to his touch by giving him his blood. It was common knowledge. Even to idiots like the Winchesters.
When Ruby was human she became a witch. When Ruby was a witch, she sold her soul for power. For magic. Dark magic. Magic that would give her opportunities and power. And it did. Until ten years after she made the deal.
When Ruby was alive, she was smart. She knew about magic. She knew how to curse things. How to bless things. And Ruby knew she didn't want to die after ten years. She wanted to live her life. So when she made the deal for her soul, she made sure the demon threw in some of her blood with it.
She made a knife. She crafted the thing in the blacksmith shop herself. She knew to make it powerful enough to kill her dealer she had to have three things.
She had to have the blood of an innocent human. Easily enough gotten. She accidentally cut her little sister and collected the blood.
She had to have the blood of a demon. And she had it. The demon probably just thought she had some weird blood fetish.
And she had to have the blood of one who had sold their soul, but was not yet demon. And that's where she came in.
With the right word, Ruby was able to craft a blade mixed with three types of blood, to kill demons. But it could kill demons, humans, and those who sold their soul. She knew it wouldn't work on those upper level demons. But who cares. She just needed to eliminate the one who would try and claim her soul.
And it worked. When the demon came to collect her soul, Ruby stabbed her. She then went to hide the dagger from everybody else. What Ruby failed to take into account were the hellhounds. They were the ones who would eventually drag her soul to hell.
But Ruby knew better. She hid her knife where nobody would find it. And it stayed there long enough for her to crawl herself out of the pit known as Hell. She knew the knife had a big destiny.