Is life not blessing or mystery but suffering in Buddhism? Just like the title my question is as simple as it gets.
Please give your thought as short as possible. I just wanna confirm my idea.
OP: Is life not a blessing or mystery but suffering in Buddhism?


All three are correct in Buddhism in my opinion. Past is mystery, present is suffering and future is blessing. Let me explain.

Life in the past is a mystery. Life in the present moment for an unenlightened being, is suffering. Life in the future is an opportunity for blessing.

Why is life in the past a mystery?

From the Tears Sutta:


  From an inconstruable beginning comes transmigration (samsara). A beginning
  point is not evident, though beings hindered by ignorance and fettered
  by craving are transmigrating &amp; wandering on. Long have you thus
  experienced stress, experienced pain, experienced loss, swelling the
  cemeteries — enough to become disenchanted with all fabricated things,
  enough to become dispassionate, enough to be released."


In this question, we find that when effluents arise, ignorance arises, and when ignorance arises, effluents arise. So, a beginning point of samsara is not evident.

In any case, according to the Parable of the Poisoned Arrow, it is not useful to try to discover the origin of all this suffering. And according to the Unconjecturables Sutta, that might drive you crazy.

Why is life in the present moment for an unenlightened being, suffering?

In MN 75:


  "Now suppose that there was a leper covered with sores &amp; infections,
  devoured by worms, picking the scabs off the openings of his wounds
  with his nails, cauterizing his body over a pit of glowing embers. The
  more he cauterized his body over the pit of glowing embers, the more
  disgusting, foul-smelling, &amp; putrid the openings of his wounds would
  become, and yet he would feel a modicum of enjoyment &amp; satisfaction
  because of the itchiness of his wounds. In the same way, beings not
  free from passion for sensual pleasures — devoured by sensual craving,
  burning with sensual fever — indulge in sensual pleasures. The more
  they indulge in sensual pleasures, the more their sensual craving
  increases and the more they burn with sensual fever, and yet they feel
  a modicum of enjoyment &amp; satisfaction dependent on the five strings of
  sensuality.


Analyzing this more closely in the Fire Sermon Sutta:


  "Monks, the All is aflame. What All is aflame? The eye is aflame.
  Forms are aflame. Consciousness at the eye is aflame. Contact at the
  eye is aflame. And whatever there is that arises in dependence on
  contact at the eye — experienced as pleasure, pain or
  neither-pleasure-nor-pain — that too is aflame. Aflame with what?
  Aflame with the fire of passion, the fire of aversion, the fire of
  delusion. Aflame, I tell you, with birth, aging &amp; death, with sorrows,
  lamentations, pains, distresses, &amp; despairs.
  
  "The ear is aflame. Sounds are aflame...
  
  "The nose is aflame. Aromas are aflame...
  
  "The tongue is aflame. Flavors are aflame...
  
  "The body is aflame. Tactile sensations are aflame...
  
  "The intellect is aflame. Ideas are aflame. Consciousness at the
  intellect is aflame. Contact at the intellect is aflame. And whatever
  there is that arises in dependence on contact at the intellect —
  experienced as pleasure, pain or neither-pleasure-nor-pain — that too
  is aflame. Aflame with what? Aflame with the fire of passion, the fire
  of aversion, the fire of delusion. Aflame, I say, with birth, aging &amp;
  death, with sorrows, lamentations, pains, distresses, &amp; despairs.


Why is life in the future an opportunity for blessing?

Although it's not possible to change one's past, it's possible to change one's future to become free from suffering, by changing how one responds in the present moment.

As written by Thanissaro Bhikkhu in "To Suffer Is an Active Verb":


  This is one of the reasons why the Buddha doesn’t have us try to go
  back into the past and ask, “What did I do to deserve an illness, a
  mental state, a situation in life?” He said that if you tried to trace
  all those things back, you’d go crazy. In fact, he said, you can’t
  trace back and find a beginning point for the ignorance that underlies
  suffering. But you can see what you’re doing to sustain it now.


From the Themes Sutta, we find that we can avoid future suffering by being aware of the consequences of our thoughts, words and deeds of the present moment:


  “And for the sake of what benefit should a woman or a man, a
  householder or one gone forth, often reflect thus: ‘I am the owner of
  my kamma, the heir of my kamma; I have kamma as my origin, kamma as my
  relative, kamma as my resort; I will be the heir of whatever kamma,
  good or bad, that I do’? People engage in misconduct by body, speech,
  and mind. But when one often reflects upon this theme, such misconduct
  is either completely abandoned or diminished.


Continuing the Fire Sermon Sutta, we find that ultimately all suffering traces back to craving and craving needs to be eliminated, in order to become free of suffering:


  "Seeing thus, the well-instructed disciple of the noble ones grows
  disenchanted with the eye, disenchanted with forms, disenchanted with
  consciousness at the eye, disenchanted with contact at the eye. And
  whatever there is that arises in dependence on contact at the eye,
  experienced as pleasure, pain or neither-pleasure-nor-pain: With that,
  too, he grows disenchanted.
  
  "He grows disenchanted with the ear...
  
  "He grows disenchanted with the nose...
  
  "He grows disenchanted with the tongue...
  
  "He grows disenchanted with the body...
  
  "He grows disenchanted with the intellect, disenchanted with ideas,
  disenchanted with consciousness at the intellect, disenchanted with
  contact at the intellect. And whatever there is that arises in
  dependence on contact at the intellect, experienced as pleasure, pain
  or neither-pleasure-nor-pain: He grows disenchanted with that too.
  Disenchanted, he becomes dispassionate. Through dispassion, he is
  fully released. With full release, there is the knowledge, 'Fully
  released.' He discerns that 'Birth is ended, the holy life fulfilled,
  the task done. There is nothing further for this world.'"