Currently, if you trigger one of the filters from a list of phrases in SpamRegex, a result will pop up that says:

The text you wanted to save was blocked by the spam filter. This is probably caused by a link to a blacklisted external site.

The following text is what triggered our spam filter: imgur.com

Screenshot:

This is ambigious, and doesn't give an exact reason. It just gives a probability: "This is probably caused by a link to a blacklisted external site." This basically says: "Hey, this is blocked because it's on the blocked list." What it doesn't say is why, which is the important question that will arise when such a thing happens during editing, that needs to be answered.

I suggest providing the whole change, which is pulled from the reason of that change in Special:SpamRegex, or displaying the full log action if and when a log gets implemented (T152164). With the whole change being showed, it provides the person who added it so that the editor can shoot up a discussion (sometimes filters aren't specific enough, and will block something off even though it shouldn't be).

This is similar to how the delete core action works: when you view an old, deleted page on a wiki, a log action from Special:Log/delete will be displayed to explain to you why that page was deleted.