No, there is no need to believe blindly in whatever the Guru said. Totapuri the Advaita Guru of Sri Ramakrishna taught him Jnana for 11 months and still Sri Ramakrishna did not give up Bhakti.
MAHIMA: "I have a question to ask, sir. A lover of God needs Nirvana
some time or other, doesn't he?"
MASTER: "It can't be said that bhaktas need Nirvana. According to some
schools there is an eternal Krishna and there are also His eternal
devotees. Krishna is Spirit embodied, and His Abode also is Spirit
embodied. Krishna is eternal and the devotees also are eternal.
Krishna and the devotees are like the moon and the stars — always near
each other. You yourself repeat: 'What need is there of penance if God
is seen within and without?' Further, I have told you that the devotee
who is born with an element of Vishnu cannot altogether get rid of
bhakti. Once I fell into the clutches of a jnani, who made me listen
to Vedanta for eleven months. But he couldn't altogether destroy the
seed of bhakti in me. No matter where my mind wandered, it would come
back to the Divine Mother. Whenever I sang of Her, Nangta would weep
and say, 'Ah! What is this?' You see, he was such a great jnani and
still he wept. (To the younger Naren and the others) Remember the
popular saying that if a man drinks the juice of the alekh creeper, a
plant grows inside his stomach. Once the seed of bhakti is sown, the
effect is inevitable: it will gradually grow into a tree with flowers
"You may reason and argue a thousand times, but if you have the seed
of bhakti within you, you will surely come back to Hari."
The Gospel of Sri Ramakrishna, Chapter 41, At Ram's House, May 23, 1885
The Jnani mentioned by Sri Ramakrishna is his Advaita Guru Totapuri.
However, there is no need to show disrespect to Guru.
Sri Ramakrishna: Therefore I say, 'Even though my Guru frequents a grog-shop, still to
me he is the embodiment of Eternal Bliss.'
The Gospel of Sri Ramakrishna, Appendix A, With Keshab at Dakshineswar, January 1, 1881
The point is that disciples think that the human personality seen by them is the Guru. Actually it is the divine power in the Guru that is the real Guru. When scriptures like Guru Gita ask one to worship the Guru they really mean the divine power within.
There is only one Guru, and that is Satchidananda. He alone is the
teacher. My attitude toward God is that of a child towards its mother.
One can get human gurus by the million. All want to be teachers. But
who cares to be a disciple?
The Gospel of Sri Ramakrishna, Chapter 5, Tha Master and Keshab, October 27, 1882
VIJAY: "Don't the teachings of the Brahmo Samaj bring men salvation?"
MASTER; "How is it ever possible for one man to liberate another from
the bondage of the world? God alone, the Creator of this
world-bewitching maya can save men from maya. There is no other refuge
but that great Teacher, Satchidananda. How is it ever possible for men
who have not realized God or received His command, and who are not
strengthened with divine strength, to save others from the
prison-house of the world?
"One day as I was passing the Panchavati on my way to the pine-grove,
I heard a bullfrog croaking. I thought it must have been seized by a
snake. After some time, as I was coming back, I could still hear its
terrified croaking. I looked to see what was the matter, and found
that a water-snake had seized it. The snake could neither swallow it
nor give it up. So there was no end to the frogs suffering. I thought
that had it been seized by a cobra it would have been silenced after
three croaks at the most. As it was only a water-snake, both of them
had to go through this agony. A man's ego is destroyed after three
croaks, as it were, if he gets into the clutches of a real teacher.
But if the teacher is an 'unripe' one, then both the teacher and the
disciple undergo endless suffering. The disciple cannot get rid either
of his ego or of the shackles of the world. If a disciple falls into
the clutches of an incompetent teacher, he doesn't attain liberation."
The Gospel of Sri Ramakrishna, Chapter 7, The Master and Vijay, December 14, 1882