If you noticed blood on your cock I would be concerned..
The facts are that receiving oral sex without a condom is not safe. Using a condom does not garentee safty either but it greatly improves the odds from no protection at all.
It's a fact you can easily get a STD from getting sucked and you can't tell by looking at someone if they got a STD or not.

While reported cases of getting more survere forms of STD such as HIV via oral sex is low compared to other methods, it can and does happen. Especially if you have any skin iretation or microscopic cuts on your penis (that you may not be aware of) or exposed to a high viral count of HIV in the suckers mouth.

Is your life and health worth it?
I use condom when I get sucked, at least over the head. I would not suck off a guy without him having a condom over the head at least.
Sure it is not as great as without a condom but its foolish to think your not taking a high risk. A lot of the guys sucking cock have multiple partners in a day and the odds of catching something is not as low as some tend to think.