I've read about half of the Koran and I've noticed several verses which seem to imply that Allah causes people to disbelieve by blinding them to the truth and also that evil is created by Allah to test the faithful. To me this is disturbing because if Allah creates evil creatures and blinds disbelievers to truth then the source of evil and disbelief is ultimately not Satan but Allah.
You have to quote the actual ayah(verse) which you are referring to, otherwise all the answers would be going around in random circles.
Allah swt also says somewhere that " you err because of your own nafs( ego/desires)".
Allah swt does not mis-guide people, it is satan who does that. You have to understand the verse with its proper meanings, and vast contexts. I fear that you have read another case of mis-translation. For what I have read, it is said something like,
" And they ( the disbelievers) are blinded by the darkness as if the light of their eyes has been snatched, and they act as blind, deaf and mute, so then, do they not see?
" And that is Implied to THOSE disbelievers specifically, who have sinned so much, have been so open in their dis-obedience to Allah, that by continuously doing so, they seal their own fate, commit shirk (polytheism, assign partners to Allah), follow the shaitan, EVEN after knowing WELL that satan will side with them only till this world lasts, after that ,they shall have no helper, no companion. It is one thing to want guidance and that is the true want/right of man, and it is another thing to not want guidance, because true guidance might not get you worldly riches/pleasures as are given by following desires which are lead by satan.
( If you have read that satan and its forces give access to immense wealth and worldly delights to its followers, yes, Only by the will of Allah, but not with His pleasure, that wealth and riches are the deception of satan, which blind their eyes and they fore-sake their creator. Thinking that this life is the only life they will have, so why not enjoy it to the fullest even if it means being corrupt and tyrant and disobeying God's laws, cuz at this point in the world, it is really really hard to be all wealthy and yet be totally pious, wealth is a temptation and all temptations can lead to fitnah ( meaning evil desires; start from any rich man, or any man in position of power, from the president of the state to a senator, IMF's Ceo etc, and one can open a never ending closet, full of stinky skeletons)) And then there are those who are given riches by Allah swt Himself, and they spend all of it while obeying him and not indulging in vain discourse and throwing money away on luxury.
And THAT is the point, where the verse carries it meaning, that one becomes so clouded by one's own sinning that he does not even wish to see the worth or value of his actions, neither does he think that he is answerable to anyone in this world or the hereafter, as it is his own personal matter. And such a man is blinded by his apparent success, fame, wealth and popularity in women, that he is a legend or something and people will follow his ways, his lifestyle and will fall for the same old worldly pleasures that he gave himself into, and he himself, along with people supporting him, live in this deception by their own free will.
For Allah swt says something like (similar words) " I have rights upon every soul and I shall keep on guiding them as long as they ask for it".
Why then people ask for innumerable wealth & riches and power and beautiful women and children, BUT Never once ask the Creator to guide them, if they are right? What is so hard about asking for true guidance if one is so sure that they are not wrongly guided?
Why fear guidance if one is so sure that it is leading to something good? The truth is, that all these people deny themselves the guidance from Allah swt, by arrogantly thinking that they don't need divine guidance and as long as they are famous, rich ,desired, they are rightly guided and Allah swt knows their thinking very well. it is aprt of Muslim prayer thats is said 5 times a day, to repeat the prayer for right guidance in each moment of it, which other religion teaches that?
He gives them trials, and chances in their lives, even rich corrupt people go through their own worries, but do they turn to God for help? They mostly don't, and when they do, it is only momentarily and later, they forget that they were helped by Him in the time of need and cant help running back to satan being his loyal aid ,embracing hell fire.
Its all free will, where the Creator is on the Man's side and man is on satan's side, and chooses to be, even if the satan disappoints him several times, he doesn't want to give up his corrupt ways/wealth. Then why choose to give up on God?
Allah swt has granted mercy and forgiveness to anyone and everyone who wishes to seek it, no matter how huge or offensive the crime a man commits, and even then people don't ask Him for His forgiveness? Is that Not blindness caused by a personal choice? Is that not arrogant and evil to be so self absorbed as to not ask to be rightly guided?
As for the assumption that Allah swt created satan and he is evil so all evil must be created by him is a blatant ignorant assumption. People who read the Quran in full detail know that Satan was not called satan initially, and was Created as a pious being among Djins, who worshiped God all over the Universe and was among His favorites. What caused his decline from that status? he committed an arrogant mistake, and did not ask for forgiveness in jealousy of man being created, despite being prompted by God to ask for forgiveness .( imagine the omnipotent God asking his creation to ask for forgiveness, despite the fact that God had all authority and full power to burn and shatter him to pieces just right there).
And from then on satan promised to corrupt the entire humanity so that humanity would disobey God as much as he did, But Allah swt told him that humans would not act as arrogant and dis-obedient as him, and He will repeatedly forgive them as long as they ask for forgiveness and guidance, except those, who choose to side by satan and act arrogant about their sins and insist on increasing in their sins while not seeking forgiveness.
Hence Allah swt places trials( shortcomings, small failures) infront of us to make us spiritually strong and satan keeps dazzling temptations infront of us to weaken our spiritual strength so that we may fall into his trap and deny our spiritual growth.
hope you got the concept of evil figured.