Why was the Trade Federation so interested in Naboo? Why did they blocked Naboo especially since it's a small planet and apparently unimportant.
Because they were coerced into doing this by Palpatine / Sidious.
Senator Palpatine hails from Naboo. He needed Naboo on the center stage in order for him to gain the much-needed sympathy votes to influence the voting for the replacement of Chancellor Valorum. To that end, as Sidious, he influenced the Viceroy of the Trade Federation to set up the blockade around -and even invade- Naboo.
From Wikipedia (https://en.wikipedia.org/wiki/Palpatine):
In The Phantom Menace, the character is depicted as a middle-aged Galactic Senator from the planet Naboo who is secretly the Sith Lord Darth Sidious. As Sidious, he influences the corrupt Trade Federation to blockade and invade Naboo. Queen Padmé Amidala of Naboo flees to the planet Coruscant to receive counsel from Palpatine, unaware that he actually engineered the invasion. After a plea for help from the senate results in bureaucratic delays, Palpatine persuades the queen to make a motion to have Supreme Chancellor Finis Valorum removed from office.