The causal agent of the passive voice does not appear to be God, but unbelief. That is, there is compelling biblical evidence that the agent of the blindness in this context (in the passive voice) was "slowness of heart."
First we see that when Jesus had earlier spoken of his imminent death and resurrection, the disciples did not understand because at that time they did not believe the Scriptures.
John 2:19-22 (NASB)
19 Jesus answered them, “Destroy this temple, and in three days I will raise it up.” 20 The Jews then said, “It took forty-six years to build this temple, and will You raise it up in three days?” 21 But He was speaking of the temple of His body. 22 So when He was raised from the dead, His disciples remembered that He said this; and they believed the Scripture and the word which Jesus had spoken.
It was not until after his resurrection that they believed. That is, unbelief blocked their understanding of the resurrection of Jesus. In the passive voice, the truth therefore became concealed or hidden from them.
Luke 9:43-45 (NASB)
43 But while everyone was marveling at all that He was doing, He said to His disciples, 44 “Let these words sink into your ears; for the Son of Man is going to be delivered into the hands of men.” 45 But they did not understand this statement, and it was concealed from them so that they would not perceive it; and they were afraid to ask Him about this statement.
Luke 18:31-34 (NASB)
31 Then He took the twelve aside and said to them, “Behold, we are going up to Jerusalem, and all things which are written through the prophets about the Son of Man will be accomplished. 32 For He will be handed over to the Gentiles, and will be mocked and mistreated and spit upon, 33 and after they have scourged Him, they will kill Him; and the third day He will rise again.” 34 But the disciples understood none of these things, and the meaning of this statement was hidden from them, and they did not comprehend the things that were said.
It was not God as the primary cause, but unbelief that blocked their full understanding until Jesus identified himself after the resurrection. Notwithstanding that the resurrected body of Jesus still bore the scars of the crucifixion (compare John 20:20 with Revelation 5:6) and in this sense his glorified body was disfigured (please click here for more discussion, s.v., Argument from Disfiguration), the PRINCIPAL cause of their lack of recognition and discernment was their lack of faith, and so they were not looking for or expecting the resurrection of Jesus.
For example, when the risen Jesus was on the road to Emmaus, he chided his disillusioned disciples in this regard with the following words:
Luke 24:25-26 (NASB)
25 And He said to them, “O foolish men and slow of heart to believe in all that the prophets have spoken! 26 Was it not necessary for the Christ to suffer these things and to enter into His glory?”
This statement was not rhetorical flourish, but an encouraging reprimand because of their lack of faith in the Scriptures. Again, the natural body of Jesus had undergone its physical transformation at resurrection (1 Cor 15:43-51), however it was still their internal bias --based on their lack of faith-- that caused the blindness of these disciples on the road to Emmaus to "see" Jesus. When they finally recognized Jesus, was it because they saw his "disfigured" hands with the nail marks as he broke the bread in front of them?
Finally, without casting moral aspersions on the weakness of faith among the disciples of Jesus (since we are no better than they), we see that an internal bias occurs when one does not accept the plain and normal reading of Scripture by faith. When we receive the Scriptures like children, the Lord reveals the wisdom of His word.
Luke 10:21 (NASB)
21 At that very time He rejoiced greatly in the Holy Spirit, and said, “I praise You, O Father, Lord of heaven and earth, that You have hidden these things from the wise and intelligent and have revealed them to infants. Yes, Father, for this way was well-pleasing in Your sight.
In conclusion, the bias from unbelief will result in "blindness," which filters out anything that contradicts that internal bias (to include the appearance of someone like Jesus who rises from the dead -- see Luke 16:31). In this sense, and in the passive voice, is the truth thus "concealed or hidden" from us so that we are "kept from seeing" and understanding the Word of God (who is Jesus). We must trust His word therefore like children.