The BLOB may be very large and if you try to move/Write it consume a lot of system memory and that for sure will affect Uipath performance.
You can use BinaryRader, BinaryWriter which exists in System.IO namespace.
But if your data is short in length then you can use like below code:
Byte BLOBData = new Byte;
BLOBData = (Byte)(System.Byte);
String s = System.Text.Encoding.UTF8.GetString(BLOBData);
Write this string data into Text file.
if data is large as i mentioned before then get the data in chunks of memory.