I *think* I know what they're talking about, but "block" has a different meaning in phonology in English and that's why I'd rather avoid it. Rules are thought of as applying in "blocks" in lexical phonology (blocks of rules - first one block of rules applies, then another), but that's definitely not what they mean here. They seem to be talking about the phonological word/phrase as a stretch of sound, or unit if you like - I've certainly heard them described as "units" in prosodic phonology often enough, but never "blocks".

I toyed with "phonic unit" at first, but I can't find any convincing references for it and also found one definition according to which it is merely a phone (much too small) - so I think I'd go with either "sound unit" or "stretch of sound".

Here are a couple of sites where they describe the phonological word/phrase as a "sound unit":

"A word has different properties depending on whether you're looking at it phonologically, morphologically, syntactically or semantically. Essentially, we end up with two different notions of word: a listeme — a sound-meaning correspondence — and a phonological word, a *sound unit* on which the spacing conventions of written English are based."
http://dingo.sbs.arizona.edu/~hharley/PDFs/WordsBook/Chapter...

"We can also consider the phonological hierarchy, which is a different, parallel hierarchy which focuses on the sound patterns in a sentence. In this, we have the additional units of syllables, which are a structural sound unit used to group phonemes together; and phonological phrases, another structural *sound unit* that groups words together within the sentence."
http://mi.eng.cam.ac.uk/~pat40/ttsbook_draft_2.pdf

And I'd precede it with "a single" (don't really think it needs "only/merely"); I'd also leave out the comma after it.

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Note added at 10 hrs (2007-04-24 21:12:33 GMT)
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An example of a phonological phrase in my textbook is:

"On Tuesdays he gives [the Chinese dishes]..."

where "Chinese" modifies "dishes". But if it means "he gives dishes to the Chinese people", it splits into two phonological phrases (because the stress pattern is different - the second syllable of "Chinese" now bears the primary stress, not the first):

"On Tuesdays he gives [the Chinese] [dishes]"

An odd example, but there you go!