I have blog on my website and I am planning to start forum. Is it okay to use forum and blog on same domain?

There is no problem use blog and forum on same domain. Both are used for making backlink for website. They are helpful for website ranking.
MijoShop Kent
Joomla Kent
If you have running blog, then I suggest to go with subdomain for forum, because there is some benefits :
>> Google will not penalize your whole site in future for low quality content, Generally, when people ask any question, and there is no answer, then it is consider as low quality webpage.
>> You can provide someone else(like developer or programmer) to control your forum without affecting to main domain.
>> You can host your forum on other server for better security. May be there are some good host that works cool with forum software like Mybb or vBulletin