That ye may be blameless (ἄμεμπτοι) and harmless, the sons of God, without rebuke, in the midst of a crooked and perverse nation, among whom ye shine as lights in the world (Philippians 2:15)
And the very God of peace sanctify you wholly; and I pray God your whole spirit and soul and body be preserved blameless (ἀμέμπτως) unto the coming of our Lord Jesus Christ.
(1 Thessalonians 5:23)
A bishop then must be blameless (ἀνεπίληπτον), the husband of one wife, vigilant, sober, of good behaviour, given to hospitality, apt to teach; (1 Timothy 3:2)
For a bishop must be blameless (ἀνέγκλητον), as the steward of God; not selfwilled, not soon angry, not given to wine, no striker, not given to filthy lucre; (Titus 1:7)
In each case, the King James translators render the Greek as blameless:
The different nuances of the Greek which are not reflected in the KJV are captured in other translations. For example the translators of the Expanded Bible have:
Then you will be ·innocent [blameless] and ·without any wrong [innocent; pure; harmless], God’s children without ·fault [blemish; C as are sacrificial animals]. ·But you are living with people that are crooked and evil [L …in the midst of a crooked and perverse generation; Deut. 32:5], among whom you shine like stars in the ·dark world [or sky; L world].
(Philippians 2:15 EXB)
Now may God himself, the God of peace, ·make you holy in every way [sanctify you completely/through and through]. May your whole self—spirit, soul, and body—be kept ·faultless [blameless] when our Lord Jesus Christ comes. (1 Thessalonians 5:23)
An overseer must ·not give people a reason to criticize him [have a good reputation; be above reproach], and he must ·have only one wife [or be faithful to his wife]. He must be ·self-controlled [sober], ·wise [have good judgment], respected by others, ·ready to welcome guests [hospitable], and able to teach. (1 Timothy 3:2 EXB)
As God’s ·managers [stewards], overseers [C probably the same church office as elder; 1 Tim. 3:1, 6, 7; 5:17] must be ·innocent of wrongdoing [blameless], ·unselfish [not arrogant/self-willed], not ·quick-tempered [easily angered]. They must not ·drink too much wine [be a drunkard], ·like to fight [be violent/a brawler], or ·try to get rich by cheating others [be greedy for gain/dishonest in business]. (Titus 1:7 EXB)
ἄμεμπτοι is an adjective and ἀμέμπτως an adverb from ἄμεμπτοι and is used only twice, both in the letter to the Thessalonians:
Ye are witnesses, and God also, how holily and justly and unblameably (ἀμέμπτως) we behaved ourselves among you that believe (1 Thesslaonians 2:10 KJV)
The difference between Philippians and Thessalonians is not the character of the individual. It is how character is perceived by others. The Philippians are encouraged to be lights to the (unbelieving) world, innocent (blameless) of fault and the Thessalonians are encouraged to be faultless (among other believers). Outsiders may call the character of the Philippians into question; so Paul encourages them to be found "innocent" when (falsely) charged.
The other two are straight-forward. Timothy is encouraged not to give people a reason to criticize and Titus is told to be innocent. This reflects challenges Titus will face:
For there are many who are insubordinate, empty talkers and deceivers, especially those of the circumcision party. (Titus 1:10 ESV)
Titus was a Gentile. This will give rise to issues for Titus which Timothy (whose mother was Jewish) will not face. Again, the difference is driven by how character is perceived by outsiders. There is an external aspect to being "blameless" which cannot be controlled. A person who is "blameless" cannot stop false accusations. However, they can be "blameless" that is innocent of the wrongs alleged by others.