The blacklevel (also called baseline) is the output of the camera or photo multiplier when not illuminated.
If a PMT is well adjusted it will be non-negative, showing a small electronic constant offset, and shifting the histogram to the right (upper values). In this case the baseline can be estimated by the lowest voxel value in the image.
A large positive black level value will reduce the effective dynamic range of your microscope, but will do no harm to the deconvolution with Huygens since it is automatically accounted for in the background estimation stage. But if your blacklevel is too high, you may reduce your Dynamic Range so much that you produce clipping in the high values (see Clipped Images), what is a loss of relevant information.
Select the image's thumbnail and go to Analysis->Statistics. Look for 'MinValue' in the Task report pane. Alternatively, you can inspect the histogram under the Histogram, Analysis->Image histogram.
The Batch Processor since version 3.3 of the Huygens Software includes an option in the Deconvolution Parameters to perform automatic blacklevel correction before proceeding with the deconvolution. Without this correction the bleaching corrector can't do a good job and will complain, because it won't be able to find a reasonable decay function for the intensity.