To find the probability of drawing a blue marble, we first need to calculate the total frequency of all the marbles.
The total frequency is the sum of the frequencies of all the colors:
\[ \text{Total frequency} = \text{Red} + \text{Purple} + \text{Blue} = 20 + 24 + 12 = 56 \]
Now, the probability of drawing a blue marble is given by the frequency of blue marbles divided by the total frequency:
\[ \text{Probability of blue} = \frac{\text{Frequency of blue}}{\text{Total frequency}} = \frac{12}{56} \]
Next, we can simplify this fraction. We can divide both the numerator and the denominator by their greatest common divisor, which is 4:
\[ \frac{12 \div 4}{56 \div 4} = \frac{3}{14} \]
Thus, the probability of drawing a blue marble is:
\[ \boxed{\frac{3}{14}} \]