Ok i might have something that helps:
Just as you get a blackscreen, start spam-clicking the minecraft window, usually you would get an error message(You can either wait a period of time too, but spam clicking is faster).
I got this error message;
Bad video card drivers!
-----------------------
Minecraft was unable to start because it failed to find an accelerated OpenGL mode.
This can usually be fixed by updating the video card drivers.
--- BEGIN ERROR REPORT 7fe0271 --------
Generated 24-9-11 14:10
Minecraft: Minecraft Beta 1.8.1
OS: Windows 7 (x86) version 6.1
Java: 1.6.0_27, Sun Microsystems Inc.
VM: Java HotSpot(TM) Client VM (mixed mode), Sun Microsystems Inc.
LWJGL: 2.4.2
[failed to get system properties (java.lang.NullPointerException)]
org.lwjgl.LWJGLException: Pixel format not accelerated
at org.lwjgl.opengl.WindowsPeerInfo.nChoosePixelFormat(Native Method)
at org.lwjgl.opengl.WindowsPeerInfo.choosePixelFormat(WindowsPeerInfo.java:52)
at org.lwjgl.opengl.WindowsDisplay.createWindow(WindowsDisplay.java:185)
at org.lwjgl.opengl.Display.createWindow(Display.java:311)
at org.lwjgl.opengl.Display.create(Display.java:856)
at org.lwjgl.opengl.Display.create(Display.java:784)
at org.lwjgl.opengl.Display.create(Display.java:765)
at net.minecraft.client.Minecraft.a(SourceFile:233)
at net.minecraft.client.Minecraft.run(SourceFile:629)
at java.lang.Thread.run(Unknown Source)
--- END ERROR REPORT b93425ef ----------
And if you DO NOT get an error message, try updating your java!
but anyhow, This error message is REALLY helpful, since as you can see, java tells me that my Video card drivers are not up-to-date.
This causes an OpenGL problem, and since minecraft uses OpenGL, it cant run!
I hope this helps, --madjack32
Edit: i took the long way, and decided that i wanted to check what video card i had(NVIDIA GEFORCE 7025),
and all i had to do was right click it and select "Update drivers".
P.S. if you take MY way, its the long way, usually you can just use the NVIDIA/Radeon control panel, but i didnt have that....