Pattern IX is used for colors and defects. Example:
اِحْمَرَّ خَالِدٌ Khaalid turned red (blushed).
اِعْوَجَّ العَصَا The stick became crooked
As for verbs with a weak middle letter (like اعوجّ ) then these are conjugated like regular pattern IX verbs. As for verbs with a weak end letter on this pattern, then I have not encountered one so far. Even though اَعْمَى is a blind person (male), I do not see in the Hans Wehr that it is conjugated in pattern IX. One could say عَمِيَ He was (or became) blind (and this is pattern I).
Likewise with the double letter verbs, I haven’t encountered one that is conjugated on pattern IX yet so I do not know if these are conjugated in pattern IX or not. Though some defects do contain double letters, ex أَصَمُّ is a deaf male, however, I did not see in Hans Wehr that it is conjugated on pattern IX. One could say صَمَّ – he became (or was) deaf – and this is pattern I .
That is not to say that being blind or deaf is a defect, this is simply something that Allah has tested some individuals with. And for those who die as Muslims pleasing their Creator, then He will give them the same as everyone else who enters Paradise (including vision and hearing) in the Next Life. And it is said that the Prophet Shu’aib was blind. Also, the Companion Ibn Umm Maktoom was blind, the late scholar Abdul Azeez Bin Baz was blind and the contemporary living scholar Ubayd al Jaabiree is blind. And it is possible there were some deaf companions and scholars as well, but in my limited knowledge, I haven’t run across that yet. One could ask the Scholars about that.
In Madinah Book 3, chapter 24 (which goes over pattern IX http://old.iu.edu.sa/spages/edu/syukbah/du4_9.htm ), it also includes the mubalaghah form for pattern IX which is اِحْمَارَّ، يَحْمَارُّ – these are two different patterns – one is increased by two letters (one alif and a double letter) and the other by three (two alifs and a double letter). According to the text of Binaa al-Af’aal, اِحْمَارَّ is the intensive for اِحْمَرَّ (refer to Binaa al- Af’aal for an example ). اِحْمَارَّ is conjugated the exact same way as اِحْمَرَّ (except with an extra alif after the meem in each conjugation : نَحْمَارُّ، تَحْمَارُّوْنَ، تَحْمَارِرْنَ etc),
(the first ر in تحماررن has kasrah)
How to conjugate verbs on pattern IX : pattern IX