What happens when a shader reaches the primitive edge and there is a
color=texture2D(texture, vec2(texCoord.x+some_positive_value, texCoord.y));
somewhere in it? I mean, what value does color get in such a call, transparent black(0,0,0,0)? There seems to be no error in doing this, but I really need to ask if this is safe to use, and are there any visible artifacts to expect. I'm making a blur shader and all tutorials I've seen use this method to access adjacent pixels.
You define what happens. What you're after is "texture wrapping"
But there's still the problem with the blur itself. There is no data outside the texture, so either you apply a wrap mode (GL_CLAMP_TO_EDGE is probably what you want) and accept there will be imperfections, or render the input to the blur slightly larger.
Possible imperfections are shown below. I've blurred a circle in gimp before and after moving it past the edge. Then filled the centre so you can see the difference better. Note the misshapen fourth circle caused by the blur operation's assumption about how the colour continues outside the border.
Just so you know texture2D applies filtering, which can be bypassed with texelFetch (note that this takes coordinates in pixels instead of normalized zero to one texture coordinates).
