I usually submit one article to any one top directory. Of late I discovered this not only helps PR buy also brings decent no of visitors daily (5 to 10 per article per day). Now I am thinking to resubmit each article to say at least ten directories. From these I am not expecting a PR benefit but they can definitely bring more visitors I feel? But my doubt is whether SEs will think this as a black hat method? Please advise me.
It is not seen as black hat; it simply prevents the same article for ranking and bringing in traffic on all but one of the article websites. Only one exact copy of the articles is given priority and the rest are filtered in results and seen as duplicates.
You may or may not still gain traffic value from the process that you are utilizing.
In my experience, if you wait long enough (a couple of weeks) you will sometimes see multiple of the same article listed on different sites appear in SERPS. It seems to me that 5-10 article directories is the limit though.
Submitting to multiple can provide an overall PageRank boost, assuming they link back to your site. However, doing this too often seems to result in the PageRank worth of the tactic diminishing.
Another option is to create slight variations in the article for each place where you upload it -- slightly different titles and content. This can work around some duplicate content filters, helping your article fill the search results more effectively.