south1 wrote:I would like to block their light to force them to negotiate.
Yes, that might be enough to get them to agree to frosted or patterned glass. But they might not want to negotiate, and if they have access to lawyers the other option is to seek an injunction for the removal of the barrier. Right to light, easements and estoppels etc. I have no idea if they would be successful, but it could get quite costly and nasty.
Have you tried talking to them about your concerns and seeking a neighbourly solution? Possibly a waste of time, but a judge will surely expect that it is attempted before more extreme measures.