It's the R

From Wikipedia (R-colored vowel):

In English, pronunciation of /r/ is difficult, and it is one of the most frequently misproduced sounds for a number of reasons including:

• It can be either consonantal or vocalic;
• There is no single defined way to produce the sound either by manner or place of articulation;
• It tends to be a later developing sound; and
• Correct pronunciation is not dependent upon spelling.

Listening to Blueprint 3 this week and thinking about Jay-Z's use of the sound and letter "R". He usually pronounces it as the non-rhotic "ah", "aw", or "uh"; sometimes pronounces it with exaggerated breathiness; sometimes using "Standard" pronunciation, but exaggerating it to still sound "wrong" (such as in the song "Haters" and "Reminder") echoing Andre 3k's similar heavy rhotacization of the phoneme in the song "ATLiens".
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Rakim "the god" has an album called 18th Letter (which is "R", to save you some counting), and a song (off a different album) called It's The R (which is the song I'm about to talk about).

Rakim is the pioneer and master of "written" hip-hop ("the paragraph ambassador").

To give some example of how he plays with letters and sounds (relating this to his godhood):

he plays with tenses of the verb "to be" and the first and last letters of his name. RakiM. The "R" as "are"; the M as "am". (He calls himself "the god"). Other times, the first two letters of his name "Ra" (Egyptian god of the sun), and the last two "I" and "M" pronounced to sound like "I am". Not only this, but just this play with "first" and "last" letters invokes the phrase "the alpha and the omega".

In the song It's The R, he spends the first verse throwing out letters, acronyms and spelling out words. The second, he forces all sorts of words to rhyme with his pronunciation of the letter "R".

Even the producer/DJ (can't find the credits right now) on this song plays with the /r/. The scratching of the main sample often sounds like an /r/ sound inserted chaotically into words. And as the the sample plays out at the very end ("I'm the most popular demanded | rap classic") the "R" in the word "rap" is scratched back and forth for several seconds.

listen: