Are the Front- and Back-Porches Required to be Black for VGA?
The answer is, yes for analog VGA, blanking is required (that is, porches set to analog black), as set out in the VESA specification for each display format.

Why? The RGB signal can be (and often is) offset from 0V, especially if sync-on-green is in use. So the receiver needs to sample analog black just after sync and use it as reference for the remainder of the line. This is required regardless of whether the device is an analog CRT or a digital LCD.

Even if TTL sync is being used, sampling blank cancels any DC offset the sending DAC or its buffer may have.

The porches also define a ‘stand-off’ area between sync and video to give time for processing circuitry (filters, drivers) to normalize. In the specific case of a CRT, this includes beam flyback time which is triggered by the leading edge of h-sync. For an LCD it may be to transfer the row buffer data to the active display.

Don’t forget that there’s vertical blanking time too, for the same reasons.

Digital interfaces like HDMI or DisplayPort don’t require blanking, and instead use that time in other ways, such as for sending audio data.