Given events $A$ and $B$ within the sample space $S$, the following formula can be used to compute conditional probabilities.
\[
P(B \mid A)=\frac{n(A \text { and } B)}{n(A)}
\]

Use this result to find the probability $\mathrm{P}$ (blacklspade) when a single card is drawn from a standard 52-card deck.
$P($ blacklspade $)=\square($ Simplify your answer. $)$

Step 1 ：The problem is asking for the probability of drawing a black spade from a standard 52-card deck. In a standard deck, there are 26 black cards and 13 spades. However, since all spades are black, the intersection of black cards and spades is just the set of spades, which is 13.

Step 2 ：We can use the formula for conditional probability to find the probability of drawing a black spade. The event A is drawing a black card, and the event B is drawing a spade. We want to find P(B|A), the probability of drawing a spade given that we have drawn a black card.

Step 3 ：According to the formula, this is equal to the number of outcomes where both A and B occur divided by the number of outcomes where A occurs. In this case, the number of outcomes where both A and B occur is 13 (the number of black spades), and the number of outcomes where A occurs is 26 (the number of black cards).

Step 4 ：Therefore, we can calculate the probability as 13/26. The final answer is \( \boxed{0.5} \).