The word "angel" in the Bible is translated from the Hebrew word "malawk" in the Old Testament and the Greek word "angelos" in the New. Both words mean "messenger." These messengers could be in the form of human or spiritual beings. For example, both Malachi 3:1 and Matthew 11:10 refer to John the Baptist and say "behold I will send my messenger (Hebrew – malawk; Greek – angelos) of the covenant." However, because many of the messages were delivered by spirit beings, it became common to refer to these spirit beings as angels. We will assume the questioner is asking about spirit beings. 

In the Bible, we could only find two instances where angels are mentioned with darkness. They are I Peter 2:4 and Jude 1:6. Both of these citations refer to a class of angels before the flood. These disobedient angels were unauthorized to materialize with human bodies. They procreated with human women. (See Genesis 6:1-7) These fallen angels are figuratively kept in chains of darkness (as a punishment) until their day of judgment. Scripturally, they are called demons, evil spirits, unclean spirits. Their way of communicating with humanity is through mediums, soothsayers, spiritualists, possession, etc. The practice of spiritism by any Israelite was strictly forbidden – the penalty being death (Leviticus 20:27).

So, what is the difference between angels of light and darkness, between holy and unholy angels (demons)? Perhaps the question we should first ask is "why do we need to differentiate?" Nowhere in the Bible are Christians instructed to communicate with angels. Instead, we are to be guided by the Bible. 2 Timothy 3:16,17 (NKJV) states "All Scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness, that the man of God may be complete, thoroughly equipped for every good work."

We know we have guardian angels. Hebrews 1:14 (NET) reads, "Are they (obedient angels) not all ministering spirits, sent out to serve those who will inherit salvation?" Thus, obedient angels are watching over and protecting the Lord's people. However, a holy angel would deem it detrimental and out of harmony with God to visibly manifest or communicate itself to Christians who must "walk by faith and not by sight." (2 Corinthians 5:7)  

If we rely on angels rather than the word of God for our guide, we make ourselves vulnerable to Satan's deceptions. Satan can transform himself into an "angel of light," (2 Corinthians 11:14). So our safest course, if we encounter any messenger/angel (angelos), regardless of form (whether delivered as a vision, a dream, an inspiration, a sign, or any other sort of revelation), we should not accept it. We can not act upon it; we should not proclaim it. Anything else is darkness.