Sorry for the very late response (just joined travel forum).
I cycled from Istanbul to Trabzon a few months ago (May 2007). The portion from Samsun onwards was along the Black Sea Coast. The roads were perfect for cycling but the scenery is not too great, however it is a very direct and easy route eastwards. Traffic is not really a problem as there is an excellent hard shoulder all the way from Samsun. However, there is a very major hazard for cyclists - tunnels (lots of them).
I'm flying back to Trabzon on August 14th to pick up my route then cycling eastwards through Georgia, Azerbaijan and Turkmenistan. Check out my blog on
www.bill2007.crazyguyonabike.com for more details of my Turkish bike ride. If anyone wants any info on cycling this part of Turkey please contact me through my website.
Hope this message reaches you in time. Good luck and enjoy yourself.
BillD