When Jesus quoted the words from Isaiah 6:9-10, He was answering His disciples why using parable. The synoptic gospels had kept the same account, but the details significantly various, that worth to pay attention to.
The Matthew account (Matt 13:11-15) contained the full text of Isaiah. Mark (Mark 4:11-12) and Luke (Luke 8:10) had it mostly summarized. So it is better to use Matthew account to understand what it meant.
11 He replied, “Because the knowledge of the secrets of the kingdom of heaven has been given to you, but not to them.
12 Whoever has will be given more, and they will have an abundance. Whoever does not have, even what they have will be taken from them.
13 This is why I speak to them in parables:
“Though seeing, they do not see;
though hearing, they do not hear or understand.
14 In them is fulfilled the prophecy of Isaiah:
“‘You will be ever hearing but never understanding;
you will be ever seeing but never perceiving.
15 For this people’s heart has become calloused;
they hardly hear with their ears,
and they have closed their eyes.
Otherwise they might see with their eyes,
hear with their ears,
understand with their hearts
and turn, and I would heal them.’ (NIV)
First of all, is it true that Jesus intentionally using parables to chose His disciples, so only His disciples understand the secret of the kingdom of heaven, and the rest, were destined to be deserted? This is certainly in conflict with God wants to save every single one, as telling in the parable of the lost sheep (Matt 18:10-14; Luke 15:3-7)
In fact, amongst 47 parables in the Gospels, Jesus only gave explanation to two; The parable of the sower (Matt 13:18-23; Mark 4:13-20; Luke 8:11-15) and the parable of the weeds (Matt 13:36-43). But there were 87 teachings of various topics that Jesus used plain words. If Jesus did explain secretly to the disciples other parables that were not recorded in the Gospels, then the disciples were very likely only understood 2 out of 47.
And therefore, it was not the Lord kept the people blind and deaf. It was their disobedience causing their heart calloused, not able to see and hear the Lord correctly.
Apparently many might miss to see there are two promises from the Lord in the narrative; verse 12 and 15b.
In verse 12, 'whoever has' refers to the people who put their faith on Jesus to follow Him. The Lord's spirit will help them to see the truth (given more), they will be given eternal life (abundance). 'Whoever does not have' is the opposite, whatever power, wealth and ownership they once possessed will eventually gone when they died. This is the promise of the Holy Spirit.
Verse 15b is the promise of forgiveness. On our repentant (turn), the Lord will forgive our sins. It simply needs to reinstate the obedience to the Lord, and ask for His forgiveness.