I'd go with the DNF and a note. I am guessing that the reason that you didn't find the cache is because once you got to ground zero you discovered that the blackberry bushes were too thick. Posting a DNF would tell other potential seekers of the cache that they might want to wait until the bushes are dormant as well. Of course, a note would do the same thing, but a DNF log makes it a little more obvious that you got to ground zero and attempted a search.
That is what I did. I will change it later when I can go back. Thx.
Actually, what I meant was a DNF log and an explanation. Many cachers won't change a DNF to a Found It if you come back later and find the cache. Each search attempt is a distinct event and can have distinct logs. For example, If you think other cachers would benefit from a DNF log posted in the summer which indicated that you didn't want to search because the blackberry bushes were too thick, then came back in winter and found it, changed the log (and log date) to a find, the fact that you DNFd the cache in the summer would be lost. Someone considering looking for the cache next summer might benefit from a log which indicates that the blackberry bushes are thick that time of year. There really isn't any good reason to change a DNF log to a found it. A DNF log doesn't count against you. It's just a factual account of your experience and part of the caches, and your geocaching history.
Good point, I hadn't thought of that. When I go back, I will make a new log. Thanks.