If you're trying to hook screenfade sent by other plugins they need to send it using emessage_*()/ewrite_*() natives.
Code:
/* These are the same as above, except that the messages sent
* are also sent to all other plugins and Metamod plugins.
* This means that if you send one of these messages, other plugins will
* be notified, which was previously impossible.
* BE CAREFUL! Using these incorrectly, or not for their intended purpose,
* could cause infinite recursion or something just as bad.
* NOTE! These natives are experimental.
*/
native emessage_begin(dest, msg_type, const origin[3] = {0,0,0}, player = 0);
native emessage_end();
native ewrite_byte(x);
native ewrite_char(x);
native ewrite_short(x);
native ewrite_long(x);
native ewrite_entity(x);
native ewrite_angle(x);
native ewrite_coord(x);
native ewrite_string(const x[]);