Imagine a regular a $2015$-gon with edge length $2$. At each vertex, draw a unit circle centered at that vertex and color the circle’s circumference orange. Now, another unit circle $S$ is placed inside the polygon such that it is externally tangent to two adjacent circles centered at the vertices. This circle $S$ is allowed to roll freely in the interior of the polygon as long as it remains externally tangent to the vertex circles. As it rolls, $S$ turns the color of any point it touches into black. After it rolls completely around the interior of the polygon, the total length of the black lengths can be expressed in the form $\tfrac{p\pi}{q}$ for positive integers $p, q$ satisfying $\gcd(p, q) = 1$. What is $p + q$?

1. **Understanding the Problem:**
   - We have a regular $2015$-gon with edge length $2$.
   - At each vertex, there is a unit circle centered at that vertex.
   - Another unit circle $S$ is placed inside the polygon such that it is externally tangent to two adjacent vertex circles.
   - Circle $S$ rolls freely inside the polygon, remaining externally tangent to the vertex circles, and turns any point it touches black.

2. **Analyzing the Movement of Circle $S$:**
   - As circle $S$ rolls around the interior of the polygon, it will touch each of the vertex circles.
   - Since $S$ is tangent to two adjacent vertex circles, it will trace a path that is part of the circumference of each vertex circle.

3. **Calculating the Angle Subtended by Each Arc:**
   - The interior angle of a regular $2015$-gon is given by:
     \[
     \theta = \frac{(2015-2) \cdot 180^\circ}{2015} = \frac{2013 \cdot 180^\circ}{2015}
     \]
   - Converting this angle to radians:
     \[
     \theta = \frac{2013 \cdot \pi}{2015}
     \]

4. **Determining the Arc Length:**
   - The arc length of the unit circle that $S$ touches is proportional to the angle subtended by the arc.
   - The total circumference of a unit circle is $2\pi$.
   - The arc length for each vertex circle is:
     \[
     \text{Arc length} = \frac{2013\pi}{2015} \cdot 1 = \frac{2013\pi}{2015}
     \]

5. **Total Length of Blackened Arcs:**
   - Since there are $2015$ vertices, the total length of the blackened arcs is:
     \[
     \text{Total length} = 2015 \cdot \frac{2013\pi}{2015} = 2013\pi
     \]

6. **Expressing the Total Length in the Form $\frac{p\pi}{q}$:**
   - The total length is already in the form $\frac{p\pi}{q}$ where $p = 2013$ and $q = 1$.
   - Therefore, $p + q = 2013 + 1 = 2014$.

The final answer is $\boxed{2014}$