I came across a sentence on grammarist.com:
Blest is an archaic form that shows up mainly in references to old, mostly poetical texts and as a poetic affectation.
I find the alternation between the two words poetic and poetical interesting. Between the pair, poetic is of course much more common. According to ODO, poetical is defined as
Relating to poetry.
Written in verse rather than prose.
Having an imaginative or sensitively emotional style of expression.
And the definitions of poetic are, well, quite similar if not the same:
relating to or used in poetry.
written in verse rather than prose.
having an imaginative or sensitively emotional style of expression.
I found a discussion on Word Reference Forum which doesn't have much information but rather makes the topic more confusing.
How do they differ in usage? What is the rationale behind the alternation of words in some texts, e.g. the Grammarist post.
Best Answer
I doubt that you'll be able to find a definitive answer on this. As you say, both have, essentially, the same definitions.
Google's Ngram Viewer does show that poetic is used much more often than poetical. Basing use on how common they are, poetic would therefore likely sound better to most people.
Speaking for myself, and this is purely subjective, I almost always use poetic. However, I might be tempted to use poetical if I was referring strictly to an actual poem. So, in literary criticism, I might use poetical form rather than poetic form.
(This is similar to me always using the word use, except in very specific, technical, circumstances where I might use utilize instead. Such as in a scientific paper that offers detailed instructions on how to employ specialized tools.)
Note that this very slight distinction of mine is along the same lines as in the quotation you gave in your question. The reason they might be used differently is to make a distinction between most common contexts and highly specialized contexts.
But, for all intents and purposes, they mean the same thing. Which you use is just a matter of style and preference.