Which spell description is correct?
The description of Fabricate states that:
You convert raw materials into products of the same material. For
example, you can fabricate a wooden bridge from a clump of trees, [...]
The spell converts raw materials into products and as stated in the description you can make a bridge from trees. That is 100% correct. Trees are considered objects and their wood is a raw material.
The description of Blight states that:
If you target a plant creature or a magical plant, it makes the saving throw with disadvantage, and the spell deals maximum damage to it. [...] If you target a nonmagical plant that isn't a creature [...]
There are two different plants in DnD as I have written below. There are the "creature" or/and "magical" plants and the "object" or "non-sentient" plants.
The creature/magical plants are, as you may have already guessed, creatures. They have stats and abilities just like any other creature in the DnD universe.
The object/non-sentient plants are, again, non-sentient. For example, the flower inside a flowerpot is not a creature. Anything that cannot do at least one of the following shouldn't be considered a creature:
Move, eat, speak, breathe, see, hear
What are plants: creatures or objects?
Let me explain...
There are plant creatures such as Myconids or Treants who breathe, walk, eat just like any other creature. These are considered creatures. You can find a list of them here.
They have stats and abilities like any other monster in D&D 5e. As stated in the description of the "plant" creature type (Monster Manual, p. 7):
Plants in this context are vegetable creatures, not ordinary flora. Most of them are ambulatory, and some are carnivorous.
There are also 'normal' plants, what we call non-sentient plants such as Trees (Ash, Birch, Oak) or flowers such as Orchids. These are considered 'objects' or non-sentient beings. You can find a list here.
Generally, any plant that doesn't come with a stat block should not be considered a creature.