Suppose $X$ is a random variable. In most undergraduate math texts, one writes the expected value of $X$ as $\text{E}X$ or $\text{E}[X]$. Similarly, the probability that $X$ is greater than some value $x$ is usually written as $\text{P}(X > x)$ or $\Pr(X > x)$.
However, in most graduate-level treatments of probability that I have read, I notice $\mathbb{E}$ and $\mathbb{P}$ are used instead.
Is there any particular reason for this change of style of notation?
[If anyone can make this a community wiki, I'd appreciate it.]
I don't know why there would be an increased usage of $\mathbb{E}$ over $\mathbf{E}$ in graduate texts, but, basically, $\mathbb{E}$ is supposed to represent the stylized way people draw bold fonts on a chalkboard (or in your notes). The only difference between $\mathbb{E}$ and $\mathbf{E}$ is the font.
Perhaps the reason for the increased usage is that there has been a fairly recent explosion in the number of graduate mathematics texts published, and while the undergraduate library has grown as well, it has grown more slowly. So you are more likely to see newer styles in books for more advanced topics.