Won't help with searches, but on your main YT home/landing page where it lists a bunch of recommended video's for you, if you move your mouse over the text title of the video three dots will appear to the right of the title. If you click on those three dots at the bottom you will have the option to either 1) Not interested or 2) Do Not Recommend This Channel. I use the latter a lot to block channels that keep showing up in my recommendations that I don't have an interest in.