I can control my blacks but yes jpegs and tiff files unless you take them back into photoshop and re do the black areas I am pretty sure your SOL, but hey the customer supplies the file so it is up to them to make them correct for print. I have a 7 page pdf file on my web site that tells them what I want for print and if they don't follow it correctly the results will be less than desirable. Having said that I still like to get our finished products to look the best we can, afterall I am not in business to do one time print jobs.