How does the poem "A Poison Tree" by William Blake support or negate that anger is an emotion that wells up inside until you feel you must spew it...

I would think that the main theme of this excellent poem,
which can be related to your question, can be clearly seen in the first stanza, which
states:

I was
angry with my friend:

I told my wrath, my wrath did
end.

I was angry with my
foe:

I told it not, my wrath did
grow.

There is a clear
division here in terms of how we deal with anger and the consequences of anger that is
not expressed and dealt with. In the first instance, the speaker, feeling angry towards
their friend, yet clearly still considering that individiual to be a "friend," tells
their friend about their anger, resulting in the end of that destructive emotion.
However, the other instance narrates how the same speaker felt anger towards his "foe"
but did not tell him about it, and thus the anger grows as a result, ending in the death
of his foe. Clearly, this presents anger as an emotion that if it is not expressed will
eventually "spew out" in your words and hurt both the object of anger and the person
feeling the emotion himself.