I know of someone who has experienced this. They was riding a 4 wheeler up in the hill, and a big black blob grabed onto his arms and tried to jerk him off of the 4 wheeler, and he had to stop. He said to the black blob leave me alone, and it did. What could that of been?

You need to be far more detailed.

Thats all that I know. One of my friends told me that happen to them, and we have been trying to figure out what it is. They said it was a big black blob and it kept grabing at his arm trying to pull him off of a 4 wheeler and he said he stoped the 4 wheeler told it to leave him alone and it was gone.

I would advise you to be extremely cautious of hearsay, regardless of how well you know the person.  I have two close friends of mine that are die hard believers in this stuff.  They have told me some very odd stories.  I do not doubt that they believe them, but I remain unconvinced. 

In this situation with your friend, I would advise that you should determine how well you know the person and how likely it is that they're just joking with you.  If you find it likely that they genuinely believe what they're saying, which I found with my two friends, I would still not advise to just take their word for it.  You can only know for certain when you have evidence and facts in front of you. 

but u can witness the event on ur own eyes, and still not have any evidence.

does it make less real?

While I agree with Mark, a few specific questions could possibly help a lot.

What time of day was it? The date could also be useful.

Where was it? Urban? Woods? Hills are everywhere.

How big was the 'blob'? Did it have any features at all? It was obviously capable of moving and holding onto someone so it couldn't have just been a pile of goo.

At the very least, the responses should help you determine whether this person is being truthful or yanking your chain. A look at their current personal situation could also provide some insight. Maybe they just want some attention?

I don't think this person  would lie about it. It was in the woods, and I am not sure of the time but he was starting out hunting, so I say close to daylight but not all the way day light. He said it was just a blob not features and he tried to make it logical and looked for trees but the area he was in had none. It was sometime in November

but u can witness the event on ur own eyes, and still not have any evidence.

does it make less real?

Seeing is not necessarily believing... What you - the observer - perceive as real is just that: your perception of "reality."

http://www.youtube.com/watch?v=PN1NAiM55hU

If human perception was foolproof, David Copperfield, David Blaine et al would be out of business. And a million fake psychics...

but u can witness the event on ur own eyes, and still not have any evidence.

does it make less real?

I would say that if only one person witnessed something with their own eyes, it does not make it absolutely real.  Like Jake said, it's all about perception.  The amount of people who witnessed the same thing determines its credibility.  But no amount of people witnessing the same thing will be absolute proof that something is real.