When analyzing a poem, it can be helpful to put what the poem contains in contrast to what one would reasonably expect to find. This is especially helpful with Blake, who seems highly self-aware of his poetic style and diction.

In English, the benchmark poetic line since at least the Renaissance has been iambic pentameter (five feet of breve/stress rhythm). This is what we see in Shakespeare's sonnets and blank verse and it is by far the most common poetic line in English. It is the natural rhythm of English speech, the natural beating of the human heart, the way we walk. It's just hard-wired into our sense of sound and rhythm.

In "Tyger," however, Blake gives us lines of trochaic feet (stress/breve), which are the opposite of iambic:

Tyger Tyger, burning bright,

In the forests of the night;

What immortal hand or eye,

Could frame thy fearful symmetry?

William Blake uses vivid imagery and diction to reveal the existence of evil in his lyric poem, "The Tyger" from Songs of Innocence.

Using careful diction to frame one of the central questions in the poem, Blake asks:

What immortal hand or eye 
Could frame thy fearful symmetry? (lines 3-4)

Using the connotation of the word "immortal," Blake references the question of the tiger's creator, keeping the word purposefully ambiguous.  He could have chosen the word 'divine' in its place, which would have clearly indicated God as the Creator, but instead by using the word 'immortal,' Blake leaves the question open to the possibility of Satan as the tiger's creator. 

He extends the possibility in lines 5 and 6 when he alludes to "skies" or "deeps," in which his usage of skies clearly indicates heaven and "deeps" represents hell.

Blake's use of imagery also enhances the meaning of the verses, offering subtle (and sometimes not so subtle!) clues as to which creator possibly made the tiger.  In line 6, the imagery of "burnt the fire in thine eyes" and then again in line 8, "what hand dare seize the fire" conjures images of flames and burning in the reader's mind.  The repeated fire and flame imagery, especially in the fourth stanza, where Blake alludes to a furnace and anvil, makes the reader suppose that the fiery tiger's creator must be Satan, and that the tiger, a fearsome predator, was forged in the flames of hell.